or 


THE  UNIVERSITY  OF 

au  of  Economic  Geology  and  Technology 
HANDBOOK  SERIES  NO.  1 


AIDS  TO  IDENTIFICATION  OF 
GEOLOGICAL  FORMATIONS 


BY 
I,    A.    UDDEN 


IRLF 


105 


l/D 


in 
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ublished  by 

Univcr-'.v  of   :.  xa; 


EXCHANGE 


THE  UNIVERSITY  OF  TEXAS 

Bureau  of  Economic  Geology  and  Technology 

HANDBOOK  SERIES  NO.  1 


AIDS  TO  IDENTIFICATION  OF 
GEOLOGICAL  FORMATIONS 


BY 
J.    A.    UDDEN 


Published  by 

The  University  of  Texas 

Austin 


CONTENTS 

Page 

Introduction 7 

Minerals  and  rocks 9 

Minerals     9 

Quartz    ., 11 

Mica 12 

Feldspar     12 

Hornblende    13 

Augite  and  pyroxene 13 

Serpentine  and  chlorite 13 

Glauconite 13 

Calcite   14 

Dolomite 15 

Gypsum  and  anhydrite 15 

Barite     16 

Pyrite  and  marcasite 16 

Hematite  and  limonite 17 

Siderite    18 

Salt    18 

Asphalt,  coal,  and  lignite 18 

Rocks 19 

Igneous  rocks 19 

Sedimentary  rocks    20 

Mechanical  sediments    21 

Chemical   sediments    21 

Organic  sediments   21 

Gravels,  conglomerates,  and  puddingstone. .  23 

Sands,  sandstones,  and  quartzites 24 

Packsand 24 

Micaceous  sandstone  24 

Ferruginous  sandstone 25 

Bituminous  sandstone   25 

Cemented  sandstone 25 

Quartzite 25 

Clay,  shale,  and  slate  (Argillites) 25 

Marl  .                                                                 .  27 


430114 


4  Contents 

Page 

Fire-clay  27 

Fuller's  earth 28 

Micaceous  clay  and  shale 28 

Ferruginous  clay  and  shale 28 

Bituminous  clay  and  shale 28 

Carbonaceous  clay  and  shale .  29 

Soapstone 29 

Gumbo 29 

Limestone  29 

Oolitic  limestone 31 

Shell  breccia  . . . : V.  32 

Chalk 32 

Lithographic  limestone  33 

Dolomite  33 

Caliche 34 

Travertine , 34 

Bituminous  limestone 34 

Gypsum  and  anhydrite  rock 35 

Rock  salt 36 

Flint  and  chert i 36 

Clay-iron-stone  37 

The  principal  formations  in  Texas 39 

Introductory 39 

Pleistocene  and  recent  formations 41 

The  Tertiary  formations 42 

The  Upper  Tertiary    42 

The  Middle  Tertiary 43 

The  Dewitt  Formation 43 

The  Fleming  clay 43 

The  Catahoula  sandstone  43 

The  Lower  Tertiary 43 

The  Jackson  formation  43 

The  Yegua  clay 44 

The  Cook  Mountain 44 

The  Mount  Selman. 44 

The  Wilcox 44 

The  Midway 44 

The  Upper  Cretaceous 45 

The  Tornillo  .  .45 


Contents  5 

Page 

The  San  Carlos  beds 45 

The  Rattlesnake  beds 45 

The  Escondido  beds 45 

The  Navarro  beds 46 

The  Anacacho  limestone 46 

The  Taylor  marl 46 

The  Austin  chalk 46 

The  Eagle  Ford 47 

The  Woodbine 48 

The    Lower    or    Comanchean    Cretaceous 

formations    48 

The  Buda  limestone 48 

The  Del  Rio  clay 48 

The  Georgetown 49 

The  Edwards  limestone 49 

The  Paluxy  sands 49 

The    Glen    Rose 49 

The  Trinity 49 

The  Jurassic  formations 50 

The  Triassic  formations 50 

The  Permian  formations. 50 

The  Pennsylvanian  formations 51 

m  The  Bend  formation 53 

The  Mississippian,  the  Devonian,  and  the 

Silurian  formations  54 

The  Ordovician  formations 55 

The  Cambrian  formations ¥.   55 

Archean  and  igneous  rocks 56 

Tables  for  identifying  minerals  and  rocks 57 

1.  General  appearance  under  a  magnifying 

glass 60 

2.  Examination  of  texture 62 

3.  Observation  of  color 63 

4.  Observation  of  streak 64 

5.  Test  for  hardness 65 

6.  Acid  test 66 

.    7.     Test  for  fumes » 68 

8.     Fire  test  .  .   69 


INTRODUCTION 


By  long  experience  many  drillers  learn  to  dis- 
tinguish, generally  quite  correctly,  the  three  prin- 
cipal classes  of  sediments  in  which  most  of  their 
work  is  done,  such  as  shale  or  clay,  sandstone  and 
limestone.  The  most  experienced  drillers  in  any 
locality  have,  also,  expert  knowledge  of  what  we 
may  call  the  "key  rocks"  of  the  field  in  which 
they  work.  They  know  the  rocks  that .  determine 
the  depth  at  which  casing  is  to  be  set,  or  the  depth 
at  which  the  tests  for  oil  or  gas  must  be  made. 
This  knowledge  of  rocks  is  mostly  taught  from 
mouth  to  mouth,  by  the  head  driller  to  his  helpers, 
entirely  without  any  other  guidance  than  the  limited 
experience  of  each  driller.  No  aid,  so  far  as  I 
know,  has  ever  been  prepared  by  geologists  to  help 
drillers  in  procuring  the  information  both  are  eager- 
ly seeking. 

With  the  extension  of  drilling  operations  to  re- 
gions of  widely  different  geological  formations  and 
conditions,  it  has  become  a  matter  of  some  urgency 
that  drillers'  logs  be  made  with  greater  accuracy 
and  uniformity  than  before.  The  only  way  to  en- 
courage improvement  in  this  direction  is  to  place  in 
the  hands  of  the  men  who  make  the  "logs,"  what 
aid  they  need.  By  the  use  of  a  few  very  simple 
devices,  available  to  all,  great  improvements  in  re- 
sults can  be  attained  without  the  least  loss  of  time.. 
The  present  paper  is  an  attempt  to  present  some 
information,  in  compact  form,  that  will  aid  prac- 
tical and  intelligent  men  to  correctly  identify  ma- 
terials explored  in  drilling  for  oil,  gas,  or  water. 
It  will  be  found  that  much  of  the  information  here 
set  forth  consists  of  elementary  methods 'and  facts 
drawn  from  mineralogy  and  geology.  Most  drillers 
are  eager  to  acquire  all  knowledge  they  can  in  these 
sciences.  They  have  exceptional  opportunities  in 


8  Handbook  of  Aids  to 

their  occupation  to  return  to  the  geological  profes- 
sion manyfold  any  little  help  that  can  be  offered 
them  of  the  kind  here  attempted. 


Identification  of  Geological  Formations 


MINERALS  AND  ROCKS 

Minerals,  when  pure,  have  a  more  or  less  constant 
chemical  composition,  and  with  some  exceptions  they 
have  definite  crystalline  form.  In  these  respects 
they  differ  from  rock  which  may  or  may  not  be 
constant  in  its  chemical  composition.  By  rock  we 
mean  any  considerable  quantity  or  large  body  of 
material  which  has  been  brought  together  into  a 
deposit  by  some  natural  process.  A  mass  of  ice 
resulting  from  the  accumulation  of  snow,  is  a  rock. 
Salt  or  calcareous  mud  precipitated  from  the  wa- 
ters of  the  sea  and  laid  down  in  beds,  forms  rock. 
Sand  and  gravel  are  rock.  When  indurated,  or 
hardened,  we  call  these  two  latter  deposits  sand- 
stone and  conglomerate,  respectively.  Many  rocks 
consist  of  mixtures  of  different  kinds  of  minerals. 
Many  sandstones  are  mixtures  of  grains  of  quartz 
sand  and  scales  of  mica,  and  many  limestones  are 
mixtures  of  calcite  and  dolomite. 

Minerals  are  classified  according  to  their  chemical 
composition,  while  rocks  are  classified  according  to 
the  processes  by  which  they  have  been  formed,  or 
the  manner  in  which  the  materials  of  which  they 
consist  have  been  brought  together. 

MINERALS 

The  number  of  minerals  which  occur  in  sufficient 
frequency  to  be  of  interest  in  formations  that  are 
extensively  drilled  are  quite  few.  Though  more  than 
a  thousand  different  mineral  species  are  known-,  less 
than  a  score  ar$  Commonly  found  in  the  forma- 
tions which  aue  encountered  by  drillers  in  this  state, 
may  be  enumerated  and  classified  as  follows; 


10  Handbook  of  Aids  to 

Siliceous  minerals: 
Quartz. 
Mica. 
Feldspar. 
Hornblende. 

Serpentine  and  Chlorite. 
Augite  and  Pyroxene. 
Glauconite. 

Calcareous,  lime-bearing,  minerals: 
Calcite. 
Dolomite. 
Gypsum  and  Anhydrite. 

Iron-bearing  minerals : 
Pyrite  and  Marcasite. 
Hematite  and  Limonite. 
Siderite,  or  clay-iron-stone. 

Sodium  chloride: 
Salt. 

Carbon  compounds: 
Asphalt. 
Coal. 
Lignite. 

Barium  compound: 
Barite. 

If  these  minerals  are  classified  on  the  basis  of 
the  acid  elements  entering  into  their  composition 
they  arrange  themselves  in  the  following  groups: 

Oxides : 
Quartz. 
Hematite. 
Limonite. 

Sulphides : 
Pyrite. 
Marcasite. 

Chloride: 
Salt. 

Silicates : 
Mica. 
Hornblende. 


Identification  of  Geological  Formations         11 

Feldspar. 
Pyroxene. 
Serpentine. 
Glaucdnite. 

Carbonates : 
Calcite. 
Dolomite. 
Siderite. 

Sulphates : 
Gypsum. 
Anhydrite. 
Barite. 

Hydrocarbons : 
Asphalt. 
Coal. 
Lignite. 

QUARTZ 

Quartz  is  the  oxide  of  an  element  called  silicon. 
It  is  the  most  common  of  all  minerals  and  occurs 
in  many  varities,  such  as  rock  crystal,  which  is  its 
purest  but  not  its  most  common  form;  chert  and 
flint;  agate;  jasper;  chalcedony;  and  a  number  of 
other  forms.  It  is  a  little  harder  than  glass,  which 
it  easily  scratches.  It  may  have,  like  most  other 
minerals,  any  color.  The  purest  varieties  are  either 
colorless  and  transparent,  or  white.  Crystals  of 
quartz  are  usually  six-sided  like  a  pencil,  and  term- 
inate in  six-sided  pyramids,  or  points,  at  one  or 
both  ends. 

Quartz  is  not  affected  by  ordinary  acids,  and  is 
not  much  changed  by  heating.  To  drillers  the  most 
familiar  form  of  quartz  is  that  seen  in  quartz  sand 
and  in  layers  of  flint  and  chert.  The  latter  occur 
in  some  limestones  and  greatly  retard  drilling  on 
account  of  the  hardness  of  this  mineral.  The  chert 
layers  in  the  Edwards  limestone  are  from  a  few 
inches  to  two  and  three  feet  in  thickness.  There 
are  also  some  chert  layers  in  the  Pennsylvanian 
limestones  and  in  the  so  called  Ellenburger  lime- 


12  Handbook  of  Aids  to 

stone.  Small  quartz  crystals  of  perfect  shape  are 
characteristic  of  the  Redbeds  of  the  Permian  in  the 
northwest  part  of  Texas.  Sand  grains  of  chalce- 
dony are  characteristic  of  some  of  the  middle  Ter- 
tiary formations,  and  have  come  to  be  spoken  of  as 
"rice  sand,"  from  a  resemblance  of  the  larger  grains 
to  polished  rice. 

MICA 

Mica  is  a  silicate  of  magnesia.  It  splits  into  ex- 
ceedingly thin  transparent  flakes,  which  are  elas- 
tic, and  will  spring  back  when  bent.  It  will  not 
crumble  when  heated  in  a  flame,  as  gypsum  will. 
It  is  not  affected  by  acid.  Some  mica  is  black  and 
some  has  a  golden  color.  It  is  generally  present  in 
several  different  kinds  of  crystalline  rocks,  from 
which  it  has  been  washed  and  mingled  with  sand 
and  silt,  afterward  often  hardened  into  sandstone 
and  shale.  Sandstones  in  the  Cretaceous,  and  also 
in  the  Pennsylvanian  and  the  Permian,  seldom  have 
much  mica;  while  the  sandstones  in  the  Triassic  of 
West  Texas  and  the  sands  and  silts  in  the  Eocene 
Tertiary  are  usually  very  rich  in  this  mineral. 

FELDSPAR 

Feldspar  is  a  silicate  of  potash,  soda  and  lime. 
There  are  several  kinds  of  feldspar.  It  has  straight 
cleavage  planes  that  often  show  a  pearly  lustre. 
Orthoclase  feldspar  has  a  pinkish  color.  It  is  not 
quite  as  hard  as  quartz,  but  decidedly  harder  than 
calcite.  It  can  be  scratched  by  hard  steel,  and 
does  not  slack  after  heating  to  redness  in  fire.  This 
mineral  is  the  principal  ingredient  in  granite  and 
some  other  igneous  rocks.  In  sedimentary  rocks  it 
is  sometimes  found  as  pebbles  in  some  conglomer- 
ates, such  as  those  occurring  in  the  Pennsylvanian 
in  north  Texas  and  in  the  Trinity  formation  in 
central  Texas,  around  the  Llano  uplift. 


Identification  of  Geological  Formations         13 
HORNBLENDE 

This  is  a  very  dark  greenish  or  almost  black  min- 
eral with  crystalline  cleavage.  .It  is  a  silicate  of 
magnesium,  iron,  manganese,  soda,  and  potash,  and 
has  about  the  same  hardness  as  feldspar.  It  occurs 
in  some  intrusives,  schists  and  gneisses,  underlying 
the  sedimentary  rocks  in  central  Texas,  and  in  the 
Trans-Pecos  country. 

AUGITE  (AND  PYROXENE) 

Augite  is  a  greenish  black  mineral.  It  is  a  sili- 
cate of  lime,  magnesia  and  iron.  It  has  straight 
cleavage  in  two  directions,  and  is  of  a  hardness 
very  slightly  below  that  of  feldspar.  This  mineral 
is  found  in  igneous  rocks  at  a  few  points  along  the 
Balcones  Escarpment  and  in  the  mountainous  coun- 
try west  of  the  Pecos.  It  does  not  occur  in  sedi- 
mentary rocks. 

SERPENTINE  AND   CHLORITE 

Serpentine  is  a  silicate  of  magnesia  and  iron 
containing  moisture.  It  has  an  oil  green  color  and 
a  hardness  varying  from  below  to  above  that  of 
limestone.  Chlorite  is  closely  related  to  serpentine 
but  contains  more  iron  and  alumina.  These  miner-r 
als  are  the  hydrated  decomposition  products  of  such 
minerals  as  augite  and  hornblende.  They  consti- 
tuted the  principal  oil-bearing  rock  in  the  Thrall 
oil  field  and  are  known  to  occur  as  altered  intru- 
sives at  other  points  along  the  Balcones  Escarp- 
ment and  in  the  Trans-Pecos  country. 

GLAUCONITE 

This  mineral  is  also  known  as  greensand.  It  is 
a  silicate  of  iron  and  potassium,  and  is  from  light 
green  to  very  dark  green  in  color.  It  will  turn 


14  Handbook  of  Aids  to 

black  on  being  heated  to  redness,  and  is  then  at- 
tracted by  a  magnet.  This  mineral  is  always  found 
in  small  rounded  grains  usually  mingled  with  sand 
and  clay.  It  occurs  in  some  of  the  Cambrian  rocks, 
in  the  Taylor  and  the  Navarro  formations  of  the 
Upper  Cretaceous,  and  in  the  Eocene  of  the  Ter- 
tiary beds.  A  well-marked  bed  of  glauconitic  green- 
sand  is  a  good  horizon  marker  in  the  oil  fields 
south  of  San  Antonio. 

CALCITE 

Calcite,  or  spar,  is  identical  in  composition  with 
pure  limestone.  In  most  limestones  this  main  ingre- 
dient is  mixed  with  some  clayey  material,  and  con- 
sists of  crystals  of  such  small  size  as  to  be  entirely 
invisible  to  the  unaided  eye.  The  rock  appears  to 
be  a  structureless  mass.  The  mineral  is  carbonate 
of  lime,  and  hence  consists  of  calcium  in  chemical 
combination  with  oxygen  and  carbon.  When  burned 
in  a  kiln  it  loses  its  carbon  and  forms  quicklime. 
Its  hardness  is  less  than  that  of  soft  iron  and  it  is 
easily  scratched  by  a  knife.  Limestones  in  which 
the  calcite  crystals  are  large  enough  to  be  readily 
made  out  by  the  naked  eye  are  often  described  as 
"crystalline  lime"  by  drillers.  Most  marble  is  of 
this  kind.  The  Edwards  limestone  in  the  Coman- 
chean  series  of  rocks  is  characterized  by  such  a 
coarse  crystalline  texture.  In  nearly  all  limestones 
places  are  apt  to  be  found  where  the  original  fine- 
grained rock  has  been  replaced  by  crystals  of  cal- 
cite of  large  size  which  are  spoken  of  as  spar.  Such 
spar  is  nearly  always  present  where  underground 
water  courses  traverse  the  formations,  and  quite 
frequently  they  are  associated  with  openings  or 
fissures  and  cavities  in  limestone.  In  the  lower  part 
of  the  Comanchean  limestones  there  are,  especially 
in  the  southwest  part  of  the  state,  one  or  two  layers, 
some  two  feet  thick,  in  which  the  crystalline  cleavage 
is  continuous  through  the  entire  thickness  of  these 


Identification  of  Geological  Formations         15 

layers  and  cuttings  made  by  the  drill  from  these  lay- 
ers seem  to  be  crushed  crystalline  calcite. 

DOLOMITE 

Dolomite  is  a  mineral  slightly  harder  than  cal- 
cite. It  is  a  carbonate  of  lime  and  magnesia,  and 
is  not  as  readily  affected  by  acid  as  limestone,  which 
is  a  carbonate  of  lime.  Dolomite  is  rare  except  as 
it  occurs  in  dolomitic  rocks  which  are  particularly 
common  in  the  Ordovician  and  the  Permian  series 
of  formations  in  Texas. 

GYPSUM  AND  ANHYDRITE 

Gysum  and  anhydrite  both  consist  of  sulphate  of 
lime.  Gypsum  contains  in  addition  some  water, 
while  anhydrite  is  free  from  combined  water.  When 
gypsum  is  heated  it  loses  its  water  and  is  said  to 
be  calcined.  In  this  state  it  has  the  same  chemical 
composition  as  anhydrite.  "Neither  one  of  these  two 
minerals  effervesces  when  subjected  to  the  action  of 
acids.  Both  are  softer  than  calcite  or  limestone  and 
can  be  carved  and  cut  with  a  knife.  The  crystals 
of  clear  gypsum,  known  as  selenite,  have  acute  and 
obtuse  angles,  and  they  have  good  cleavage  in  the 
direction  of  their  length  and  breadth.  They  will 
split  into  thin  leaves,  which  can  readily  be  bent, 
but  which  will  not  spring  back  to  shape,  as  the 
thin  leaves  of  mica  do.  On  account  of  its  resem- 
blance to  mica,  crystalline  gypsum  is  often  incor- 
rectly called  "isinglass."  Anhydrite  differs  from 
gypsum  in  its  cleavage.  It  breaks  straight  in  three 
directions  which  are  at  right  angles  to  each  other. 
Hence  cuttings  of  anhydrite  often  have  sharp,  square 
angles. 

Gypsum  occurs  in  many  clays  and  shales  as  sep- 
arate crystals.  It  frequently  has  formed  from  the 
disintegration  of  pyrite  in  marls  and  other  calca- 
reous material.  It  is  usually  rare  at  depths  below 


16  Handbook  of  Aids  to 

four  or  five  hundred  feet.  Below  this  depth  anhy- 
drite mostly  takes  its  place.  This  mineral  occurs 
in  extensive  beds  in  association  with  the  cap  rock 
in  our  oil  fields  of  the  coast,  and  it  underlies  ex- 
tensively the  redbeds  of  northwest  Texas. 

BARITE 

Barite  is  barium  sulphate.  It  is  white  or  color- 
less and  has  about  the  same  hardness  as  calcite,  be- 
ing easily  scratched  with  soft  iron.  It  is  much 
heavier  than  calcite.  If  some  cuttings  of  barite 
be  thrown  on  the  fire  of  a  forge,  they  give  a  pale 
yellowish-green  color  to  the  flame.  Barite  has 
been  found  in  considerable  abundance  in  some  of  the 
salt  dome  oil  and  sulphur  fields  on  the  Gulf  Coast, 
and  it  is  occasionally  encountered  in  the  Permian 
redbeds  and  in  the  Comanchean  limestones. 

PYRITE  AND   MARCASITE 

Pyrite  is  an  easily  recognized  mineral.  It  has  a 
yellow,  metallic  color*,  frequently  confused  with  that 
of  gold,  is  hard  enough  to  scratch  glass,  and  gives 
sparks  when  hit  with  steel.  It  crystallizes  mostly 
in  cubes  and  octahedrons.  Frequently  also  its  crys- 
tals present  five-sided  flat  surfaces.  Consisting  of 
iron  and  sulphur,  it  is  natural  that  this  mineral 
should  give  a  sulphur  odor  when  heated  or  when 
violently  struck  with  some  substance  of  sufficient 
hardness.  One  variety  of  pyrite  often  found  in  dark 
clays  is  known  as  marcasite.  This  is  usually  almost 
silver  white. 

Pyrite  and  marcasite  are  very  common  in  clays, 
shales,  and  slates  of  all  colors  except  red  and  yel- 
low. In  fact,  many  red  clays-  have  originally  had  a 
dark  color  and  they  have  become  red  by  the  ox- 
idation of  disseminated  pyrite.  Pyrite  also  fre- 
quently occurs  in  limestone  and  in  sandstone.  When 
evenly  disseminated  in  such  rocks  and  in  micro- 


Identification  of  Geological  Formations         17 

scopic  particles,  and  when  present  in  sufficient  quan- 
tity, it  gives  them  a  dark  color,  or  may  cause  them 
to  become  even  black.  Water  obtained  from  such 
strata  usually  has  the  odor  of  hydrogen  sulphide, 
or  of  "rotten  eggs." 

This  mineral  has  a  very  general  distribution  in 
nearly  all  formations.  It  occurs  in  grains  too  small 
to  be  seen  with  the  naked  eye  and  in  grains  and 
crystals  up  to  masses  several  feet  in  diameter.  It 
is  even  known  to  occur  in  extensive  irregular  lay- 
ers several  feet  in  thickness.  It  is  especially  apt  to 
occur  at  contacts  between  different  formations.  Lay- 
ers of  this  kind  have  been  reported  from  the  base 
of  the  Tertiary  formations  in  south  and  east  Texas. 
They  are  very  hartd  and  exceedingly  resistant  to 
the  drill.  In  coal-bearing  and  lignite-bearing  beds, 
pyrite  frequently  occurs  in  concretions  or  loaf-like 
bodies,  which  lie  flat  with  the  strata.  In  the  Aus- 
tin Chalk  this  mineral  often  forms  round  balls  from 
one  to  two  inches  in  diameter;  the  outer  surface  of 
which  everywhere  exhibits  crystalline  faces.  In  the 
Del  Rio  clay  and  in  the  Eagle  Ford  shale,  pyrite 
mostly  occurs  in  irregular  clusters  of  cubic  crystals. 

HEMATITE  AND  LIMONITE 

Red  and  yellow  ochre  (hematite  and  limonite) 
bear  somewhat  the  same  relation  to  each  other*  as 
do  gypsum  and  anhydrite.  Both  are  oxides  of  iron. 
In  yellow  ochre  this  oxide  is  combined  with  water; 
while  red  ochre  contains  no  water  in  combination. 
Both  are  usually  soft  and  are  mostly  found  only 
in  the  upper  few  hundred  feet  or  more  of  deep  ex- 
plorations. They  are  for  the  most  part  derived 
either  from  siderite  or-  from  pyrite  by  oxidation.  A 
test  which  these  minerals  have  in  common  with 
pyrite  and  siderite  is  that  they  become  magnetic 
after  being  heated  to  redness.  They  form  the  col- 
oring matter  in  most  red  and  yellow  rocks.  In 
east  Texas  extensive  beds  of  hematite  and  limonite 


18  Handbook  of  Aids  to 

occur  in  the  Tertiary  sediments.  Like  pyrite  these 
minerals  sometimes  characterize  zones  of  contact  be- 
tween formations  of  different  ages. 

SIDERITE 

Siderite  is  a  carbonate  of  iron.  In  the  impure 
state  in  which  it  mostly  occurs  it  is  called  clay-iron- 
stone. In  hardness  it  slightly  exceeds  limestone. 
It  is  also  heavier  than  limestone.  It  effervesces 
slowly  in  acid  and  becomes  magnetic  after  ignition. 
Clay-iron-stone  has  the  texture  and  color  of  a  dark 
fine-grained  limestone.  It  is  the  most  easily  smelted 
iron  ore. 

This  mineral  occurs  mostly  in  so  called  concre- 
tions, which  are  loaf-like  and  range  in  size  from 
the  smallest  up  to  several  feet  in  diameter.  These 
concretions  are  known  as  turtle-stones,  and  are  most 
frequent  in  association  with  beds  of  coal  and  lignite. 
They  are  frequently  found  in  marly  formations  such 
as  the  Taylor  Marl  and  the  marls  in  the  Midway 
formation  of  the  Eocene  Tertiary.  Drillers  very 
generally  call  them  "boulders"  when  they  are  en- 
countered in  borings. 

SALT 

Salt  is  so  generally  known  that  it  needs  hardly 
to  be  mentioned  here.  It  crystallizes  in  cubes  and 
readily  dissolves  in  water.  Dissolved  in  the  ground 
water  it  occurs  generally  in  the  Pennsylvanian  and 
the  Permian  strata.  On  the  Gulf  Coast,  it  occurs 
in  enormous  masses  in  association  with  the  coastal 
oil  fields,  and  in  the  Permian  of  the  west  it  under- 
lies in  several  extensive  beds  the  Panhandle  and  the 
Llano  Estacado. 

ASPHALT,  COAL  AND  LIGNITE 

All  these  minerals  are  compounds  of  carbon  and 
are  always  black.  They  all  burn.  Asphalt  readily 


Identification  of  Geological  Formations         19 

fuses  when  heated  and  gives  off  an  easily  recognized 
odor.  Coal  and  lignite  burn,  but  do  not  fuse  as 
readily  as  asphalt.  Particles  of  coal  or  lignite  may 
be  identified  by  roasting  them  on  a  red  hot  tin 
plate  in  a  flame,  when  they  glow  for  a  while.  Asphalt 
occurs  impregnating  many  limestones  and  filling  cav- 
ities and  veins  in  limestones  and  shales.  In  some 
sandstones  asphalt  occurs  as  an  impregnation  which 
holds  the  grains  together.  Coal  and  lignite  occur 
in  thin  layers  or  thick  seams  and  some  kinds  of 
lignite  are  parts  of  trunks  of  trees  imbedded  in 
shales  and  sandstones. 

ROCKS 

Rocks  are  classified  into  two  principal  groups: 
igneous  and  sedimentary.  Igneous  rocks  are  formed 
by  the  cooling  and  congelation  of  molten  materials. 
These  may  have  welled  out  on  the  earth's  surface 
or  may  have  been  forced  up  into  strata  that  form 
the  bedrock  overlying  the  region  where  the  fusion 
occurred.  Sedimentary  rocks  have  been  formed  by 
the  natural  and  slow  accumulation  of  sediments  of 
any  and  all  kinds,  either  in  the  sea  or  on  land. 
Rocks  of  this  latter  kind  underlie  by  far  the  larger 
part  of  the  State  of  Texas,  and  they  are  of  chief 
interest  to  drillers  in  the  southwest. 

IGNEOUS  ROCKS 

The  only  places  where  igneous  rocks  have  been 
encountered  by  drillers  in  this  state  are  in  the  Cen- 
tral Mineral  Region  in  Llano,  Burnet,  Mason,  Gil- 
lespie,  and  Blanco  counties;  along  the  structure 
called  the  Balcones  Escarpment,  in  Kinney,  Uvalde, 
Medina,  Travis,  and  Williamson  counties;  and  in 
the  mountainous  country  west  of  the  Pecos.  No 
igneous  rocks  are  likely  to  be  encountered  by  drill- 
ing in  any  other  part  of  the  state.  It  has  recently 
been  claimed  that  some  igneous  rock  has  been  found 


20  Handbook  of  Aids  to 

in  a  deep  boring  on  the  Gulf  Coast,  but  the  full  evi- 
dence of  this  has  not  yet  been  furnished.  The  in- 
trusives  in  the  Balcones  belt  are  practically  all  of 
one  and  the  same  kind,  a  very  tough,  hard  black 
basalt,  consisting  mostly  of  augite  or  hornblende. 
In  many  places  this  rock  has  been  altered,  so  that 
it  is  a  very  easily  drilled  green  chlorite  or  serpentine 
rock,  softer  than  limestone.  Such  was  the  main 
oil-bearing  rock  in  the  Thrall  oil  field.  In  the  Cen- 
tral Mineral  Region,  the  chief  igneous  rock  is  gran- 
ite. This  consists  of  pink  feldspar,  with  some  quartz 
and  mica.  There  are  here  also  extensive  areas  un- 
derlain by  gneiss  and  schist.  Originally  this  gneiss 
and  schist  was  sedimentary  rock,  but  this  has  been 
changed  by  heat  and  pressure.  It  is  now  crystalline 
and  in  part  resembles  granite.  It  usually  contains 
more  mica  than  the  granite  and  also  contains  horn- 
blende. The  igneous  rocks  of  the  mountainous  coun- 
try west  of  the  Pecos  are  of  many  kinds,  and  can 
not  be  briefly  described.  They  belong  for  the  most 
part  to  the  siliceous  types,  and  range  in  texture  from 
granite  through  porphyries,  basalts,  and  lavas  to 
obsidian,  or  natural  glass. 

SEDIMENTARY  ROCKS 

The  sedimentary  rocks  may  be  classified  into  three 
main  groups:  (1)  mechanical  sediments,  which  are 
by  far  the  most  common;  (2)  chemical  sediments, 
which  have  been  precipitated  from  solutions  in  the 
ocean  and  in  lakes;  and  (3)  accumulations  of  the 
remains  of  animals  arid  plants,  usually  called  or- 
ganic sediments,  as  animals  and  plants  both  are 
organisms. 

The  following  tables  give  a  general  view  of  the 
classification  of  sedimentary  rocks. 


Identification  of  Geological  Formations         21 


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22  Handbook  of  Aids  to 

In  nature,  the  processes  by  which  these  different 
rocks  are  made,  seldom  operate  singly  and  the  result 
is  that  few  if  any  sedimentary  rocks  are  absolutely 
pure.  While  it  is  very  instructive  for  us  to  under- 
stand these  classifications,  it  is  to  be  remembered 
that  few  of  these  deposits  are  pure  in  nature;  that 
is,  they  are  not  always  exclusively  either  one  kind 
or  another.  In  practice  it  is  misleading  to  regard 
some  rocks  as  belonging  strictly  to  only  one  of  these 
groups.  It  is  only  exceptional  conditions  that  have 
caused  nature  to  make  anything  but  mixtures.  Sand 
is  often  mixed  with  some  material  that  goes  to  make 
up  limestone.  Rock  salt  is  in  places  found  mixed 
with  clay  and  fine  sand.  Fragments  of  shells  or 
crinoid  stems  may  be  mingled  with  other  calcareous 
material  or  with  clay.  Calcareous  material  may  be 
mixed  with  clay,  and  clay  is  often  mixed  with  cal- 
careous material.  The  latter  mixture  is  so  frequent 
that  it  has  a  distinctive  name  and  is  known  as  marl. 
Coaly  material  may  be  mixed  with  clay  to  such  an 
extent  that  it  should  be  called  coaly  shale  or  "bone 
coal."  A  sandstone  may  be  impregnated  with  as- 
phalt, in  which  case  it  is  called  asphaltic  sandstone. 
Mixtures  of  these  kinds  are  so  common  that  it  be- 
comes important  to  exercise  caution  in  making  cor- 
rect determinations  of  sedimentary  rocks,  as  when 
two  or  three  ingredients  are  mingled  in  nearly  equal 
quantities  in  the  same  rock. 

It  is  also  true  that  some  sedimentary  rocks  can 
be  formed  by  either  one  of  the  three  processes  named. 
Limestone,  one  of  the  most  common  of  rocks,  may 
be  entirely  the  result  of  mechanical  action  or  silting 
in  water;  it  may  be  formed  almost  exclusively  by 
precipitation  out  of  a  solution,  or  it  may  result  from 
the  growth  of  organic  material.  Thus  some  lime- 
stones must  be  classified  as  mechanical  sediments; 
others  as  chemical  sediments;  and  still  others,  as 
organic  sediments. 


Identification  of  Geological  Formations         23 

GRAVELS,  CONGLOMERATES,  AND  PUDDINGSTONE 

In  running  and  turbulent  waters,  as  in  streams 
and  on  shores,  all  finer  sediments  are  carried  away. 
But  such  waters  in  many  places  have  built  up  large 
accumulations  of  materials  of  coarse  grain,  such 
as  gravel  and  sand.  Deposits  of  this  kind  are  in 
time  consolidated  into  conglomerate  and  sandstone, 

Gravel  rarely  occurs  in  deposits  of  any  great 
thickness,  such  as  to  approach  a  hundred  feet  or 
more.  Much  gravel  is  of  comparatively  recent  or- 
igin and  occurs  in  the  latest  sediments  which  are 
found  in  filled  river  channels  in  eastern  and  central 
Texas.  In  the  region  of  the  Panhandle  and  the 
Staked  Plains  there  are  quite  extensive  sheets  of 
gravel  in  the  Pleistocene  and  Tertiary  deposits.  These 
are  frequently  water-bearing.  Some  of  our  depos- 
its of  gravels  are  found  on  the  uplands  as  well  as 
along  stream  channels  in  the  central  and  northern 
part  of  the  state.  Even  the  most  'recent  gravels  are 
frequently  cemented  with  lime  so  as  to  form  quite 
solid  rock,  or  conglomerate. 

Conglomerates,  which  are  indurated  gravels,  oc- 
cur sparingly  in  the  Pleistocene  and  Tertiary  on  the 
Gulf  Coastal  Plain.  Sheets  of  conglomerates  con- 
sisting mostly  of  flint  occur  in  the  Pennsylvanian, 
especially  of  northern  Texas.  A  few  of  these  con- 
glomerates contain  considerable  feldspar  in  the  form 
of  pebbles,  and  drillers  have  sometimes  reported 
these  as  granite,  feldspar  being  present  as  the  prin- 
cipal ingredient  in  granite.  Such  rock  should  be 
classified  as  conglomerate,  whatever  the  composi- 
tion of  the  pebbles  happens  to  be.  Conglomerates 
are  quite  frequent  in  the  Triassic  formation  in  the 
northwestern  part  of  the  state.  These  conglomer- 
ates nearly  always  contain  well-rounded  pebbles  of 
quartz  of  a  yellowish  color,  and  they  are  in  places 
characterized  by  being  impregnated  with  black  oxide 
of  manganese.  Most  of  the  Triassic  conglomerates 
give  good  water.  One  conglomerate,  quite  like  .the 


24  Handbook  of  Aids  to 

Triassic  conglomerates,  occurs  in  the  Permian  red- 
beds  in  Tom  Green,  Coke,  Runnels  and  Taylor  coun- 
ties. Conglomerates  consisting  mostly  of  small  peb- 
bles of  white  quartz  mingled  with  sand  and  lime,  oc- 
cur in  the  Trinity  sand. 

Puddingstone  is  rare  in  the  southwest,  being  found 
only  in  some  ancient  schists  and  gneisses. 

SANDS,    SANDSTONES,    AND    QUARTZITES 

Sand  and  sandstone  present  a  great  variety  of 
rocks.  They  consist  of  more  or  less  rounded  grains 
which  have  been  worn  to  their  present  size  and  form. 
When  these  are  entirely  incoherent,  we  have  sand. 
In  such  case,  the  cuttings  will  consist  of  loose  grains 
all  separated  from  each  other.  When  the  deposit 
has  become  somewhat  more  indurated,  the  cuttings 
will  frequently  show  fragments  consisting  of  grains 
that  adhere  to  each  other,  and  we  call  the  rock 
sandstone.  The  grains  are  usually  held  together 
either  by  a  greater  or  less  quantity  of  calcareous 
material,  or  of  oxide  of  iron,  which  are  usually  pres- 
ent as  cement. 

In  rare  cases  the  sand  grains  are  cemented  to- 
gether by  pyrite.  Such  sandstones  are  usually  black 
and  when  ground  up  by  the  drill  they  cause  the  wa- 
ter to  become  inky. 

The  fineness  of  the  grain  is  spoken  of  as  the  tex- 
ture of  sandstones.  The  variation  in  this  respect  is 
considerable.  Some  of  the  sandstones  in  the  Trinity 
formation  consist  of  fairly  fine  grains  and  are  soft. 
These  are  known  to  drillers  as  pack  sand. 

In  more  than  ninety  per  cent  of  all  sandstones  the 
grains  consist  of  quartz.  In  some  sandstones  the 
grains  are  calcareous.  Such  is  the  Anacacho  lime- 
stone in  the  Upper  Cretaceous. 

Micaceous  sandstones  are  such  as  contain  mica. 
Standstones  of  this  kind  are  usually  fine-grained. 
The  Triassic  sandstones  in  northwest  Texas  are  as 
a  rule  highly  micaceous.  Sandstones  occurring  in 


Identification  of  Geological  Formations         25 

the  Pennsylvanian  and  Permian  are  almost  free 
from  mica  in  the  central  part  cf  the  state.  West  of 
the  Pecos  mica  is  more  frequent  in  the  Pennsylvan- 
ian sandstones  than  farther  east.  Sandstones  of 
the  Cretaceous  in  the  southwest  are  almost  devoid 
of  mica. 

Ferruginous  sandstone  contains  limonite  or  hem- 
atite, which  colors  them  red.  Red  sandstones  of 
this  kind  are  most  common  in  formations  that  con- 
tain coal  or  lignite,  and  also  in  formations  that 
contain  salt,  as  in  the  Permian  in  this  state.  The 
ferruginous  material  has  been  in  most  cases  in- 
troduced by  water  flowing  through  the  sands,  long 
after  the  sands  were  made. 

Sandstones  are  usually  more  or  less  porous.  When 
these  porosities  have  been  filled  with  asphaltic  ma- 
terial they  are  called  bituminous  sandstones.  When 
the  porosities  are  filled  with  oil  they  are  spoken  of 
as  oil  sands,  and  when  filled  with  gas,  they  are 
called  gas  sands.  When  they  are  filled  with  water, 
they  are  called  water  sands;  when  entirely  empty, 
they  are  called  dry  sands;  and  when  filled  with  cal- 
careous material  we  speak  of  them  as  cemented  sand- 
stone. 

Borne  sandstones  have  been  so  greatly  hardened 
by  pressure  or  by  infiltration  of  siliceous  material 
that  they  will  break  through  the  grains  rather  than 
around  them.  This  is  the  distinguishing  feature  of 
quartzite.  Reck  of  this  kind  occurs  in  association 
with  some  schist  and  gneiss  in  the  region  of  the  an- 
cient altered  rocks  in  the  central  part  of  Tex?s.  and 
also  locally  in  Tertiary  sands  on  the  Gulf  Coast 
Plain.  The  Trinity  Sand  has  also  been  changed  to 
quartzite  in  a  few  widely  separated  areas  of  very 
limited  extent. 

CLAY,  SHALE  AND  SLATE    (Argillites) 

Argillites  is  a  general  name  used  to  include  a  large 
group  of  sedimentary  deposits,  which  when  first 


26  Handbook  of  Aids  to 

formed  were  mud-like  or  clay-like  in  nature.  They 
consist  of  the  silt  and  clay  brought  into  the  sea  and 
other  waters  by  rivers  and  smaller  streams.  These 
materials  finally  settle  on  the  bottom  of  the  waters. 
Rocks  resulting  from  such  deposits  consist  of  min- 
eral particles  of  small  size,  from  the  very  finest  sand 
down  to  the  smallest  infinitesimal  sizes.  The  coarser 
argillites  may  be  described  as  silty  or  even  sandy 
clays,  or  shales.  In  mineral  composition  there  is 
also  considerable  variation  in  this  class  of  sediments. 
They  differ  from  sands  and  sandstones  in  containing 
more  aluminum-bearing  material  and  less  of  silica. 
Sands  consist  mostly  of  quartz  while  clays  are  large- 
ly derived  from  the  more  easily  crushed  and  softer 
minerals,  such  as  feldspar,  and  hornblende,  and  from 
the  insoluble  residues  of  limestone  and  other  disin- 
tegrated rock. 

The  distinction  between  clay,  shale,  and  slate  is 
based  on  hardness.  Clay  is  soft  and  plastic.  All 
argillites,  when  first  deposited,  have  been  clay.  In 
time  clay  will  harden  under  the  burden  of  thousands 
of  feet  of  overlying  formations.  All  argillites  of 
the  latest  age,  the  Quaternary,  are  yet  soft  clay. 
Argillites  of  Tertiary  age  are  in  the  condition  of 
clay,  except  in  mountainous  regions  where  they  haVe 
been  subjected  to  heat  and  pressure.  The  same  is 
true  to  a  lesser  degree  of  the  argillites  of  the  Cre- 
taceous, the  Jurassic  and  the  Triassic  ages,  which 
are  still  older  than  the  Tertiary.  In  the  Permian 
and  the  Pennsylvanian  and  other  yet  earlier  Pal- 
eozoic formations,  most  clays  have  been  hardened  to 
a  condition  we  find  in  shale,  and  in  mountainous 
regions  this  has  acquired  the  yet  greater  hardness  we 
find  in  slate.  In  the  crystalline  rocks  of  the  Archean 
formations  nearly  all  clay  has  been  changed  to  the 
condition  we  designated  as  slate  or  to  some  rock 
still  harder  than  this. 

Of  all  rocks,  clays  and  silts  are  most  easily  pen- 
etrated by  the  drill.  Three  and  even  four  hundred 
feet  of  hole  have  sometimes  been  made  in  twenty- 


Identification  of  Geological  Formations         27 

four  hours  through  such  deposits.  But  at  depth, 
very  fine-grained  clays  and  sometimes  silty  clays 
cause  much  trouble  by  "caving."  Some  clays  which 
cave  are  the  so  called  "joint  clays"  or  "slip  clays," 
which  are  cut  by  slippery  joints  that  cause  the 
ground  to  give  way  and  fall  into  the  hole  from  its 
walls.  Some  fire  clay  is  of  this  kind.  These  clays 
are  specially  apt  to  be  found  for  some  distance  be- 
low seams  of  coal  and  lignite  and  below  layers'  of 
very  black,  laminated  and  resistant  shale,  often  er- 
roneously called  "slate."  Some  caving  shale  breaks 
into  long  and  slender  splinters  and  this  has  by  some 
been  called  "pencil  cave"  and  "shoe  peg"  shale.  The 
best  practical  distinction  between  clay  and  shale  is 
to  classify  as  ?lay  only  such  material  as  is  nearly 
entirely  soft  and  plastic  and  to  regard  as  shale  any 
argillite  which  to  some  considerable  •.  extent  comes 
up  from  the  boring  in  solid  broken  fragments. 

Owing  to  the  presence  or  prevalence  of  certain 
mineral  ingredients,  different  kinds  of  clay,  shale 
and  slate  are  to  be  noted. 

Marl 

A  clay  may  be  described  as  calcareous,  when  it 
contains  sufficient  calcareous  material  to  cause  slow 
effervescence  on  the  application  of  acid.  A  clay 
containing  a  considerable  amount  of  calcareous  ma- 
terial is  called  marl,  which  may  or  may  not  also 
contain  some  red  or  yellow  oxide  of  iron.  Nearly 
all  the  clay  formations  of  Cretaceous  age  in  this 
state  are  marly.  Shale  and  clay  in  the  Pennsylva- 
nian  and  the  Permian  are  quite  commonly  devoid 
of  a  calcareous  ingredient.  In  the  Pleistocene  de- 
posits we  find  both  marls  and  non-calcareous  clays. 

Fire-clay 

Fire-clay  is  free  from  lime,  as  a  rule.  It  occurs 
in  beds  which  were  at  one  time  exposed  and  leached 


28  Handbook  of  Aids  to 

at  the  surface  and  later,  perhaps,  buried  under  peat 
which  since  has  changed  to  lignite  or  coal.  In  tex- 
ture, fire-clay  is  quite  variable,  from  the  finest  clay 
to  silty  and  quite  sandy  clay.  Fire-clays  occur  es- 
pecially in  coal  and  lignite-bearing  formations,  such 
as  the  Pennsylvanian,  parts  of  the  Eocene  Tertiary, 
and  the  coal-bearing  beds  of  the  Cretaceous  on  the 
Rio  Grande. 

Fuller's    Earth 

Strata  of  white  kaolin-like  clay  occur  in  the  lower 
part  of  the  Taylor  marl  and  .at  several  levels  in  the 
early  and  middle  Tertiary.  These  strata  are,  for 
the  most  part,  deposits  of  volcanic  dust,  which  has 
been  hydrated,  and  which  are  known  in  some  cases 
to  be  suitable  for  use  as  fuller's  earth.  Fuller's 
earth  is  also  found  in  the  Tertiary. 

Micaceous  Clay  and  Shale 

The  shales  and  clays  of  relatively  coarse  texture, 
such  as  silts,  sometimes  contain  much  mica  and  may 
be  called  micaceous  shales  and  clays. 

Ferruginous  Clay  and  Shale 

In  the  Permian  of  northwest  Texas  much  clay, 
shale,  and  silt  has  a  red  color  due  to  the  presence 
of  oxide  of  iron.  These  deposits  have  long  been 
known  as  the  "red  beds,"  owing  to  their  red  color., 
Red  clays  and  shale  occur  also  in  the  Pennsylvanian 
as  well  as  in  deposits  of  other  ages. 

Bituminous  Clay  and  Shale 

This  name  is  applied  to  argillaceous  deposits  con- 
taining bituminous  materials  such  as  oil  or  asphalts. 
They  are  nearly  always  of  a  dark  color  or  even  black, 
and  generally  give  "rainbow"  colors  and  even  strong- 
er showings  of  oil,  when  explored  in  drilling. 


Identification  of  Geological  Formations         29 
Carbonaceous    Clay    and    Shale 

Shales  containing  coaly  material  in  a  finely  com- 
minuted state  are  dark  or  even  black  and  grade  into 
impure  coal,  "bone  coal."  Shales  of  this  kind  are 
common  in  the  Pennsylvanian  formations,  and  also 
occur  in  association  with  lignite  in  the  Eocene  Ter- 
tiary. 

Soaps  tone 

This  name  is  often  incorrectly  given  to  light-col- 
ored shale.  No  soapstone  is  found  in  the  sedimen- 
tary rocks.  The  rock  known  by  this  name  is  a  result 
of  metamorphism  and  occurs  mostly  only  in  asso- 
ciation with  crystalline  rocks. 

Gumbo 

Any  clay  which  will  "gum"  or  adhere  to  the  drill, 
is  called  gumbo.  The  term  is  in  general  use  mostly 
among  drillers  on  the  Gulf  Coast. 

LIMESTONES 

A  limestone  is  a  rock  that  is  composed  chiefly 
of  carbonate  of  lime.  Rocks  composed  of  carbonate 
of  lime  and  of  magnesia  are  known  as  magnesian 
limestones,  or  dolomites.  The  amount  of  magnesia 
in  a  dolomite  varies  considerably,  from  eight  to  thirty 
per  cent.  Almost  all  limestone  contains  a  small  in- 
gredient of  clay  and  siliceous  material.  The  purest 
limestones  have  less  than  one  per  cent  of  these  in- 
gredients. In  some  limestones  they  may  amount  to 
as  much  as  twenty  per  cent,  or  even  more.  A  cal- 
careous rock  that  contains  more  clay  than  carbonate 
of  lime  or  magnesia  is  usually  called  a  marl,  especi- 
ally when  not  indurated. 

The  usual  color  of  limestone  is  light  gray  but  a 
limestone  may  have  any  color,  from  the  many  hn- 


30  Handbook  of  Aids  to 

purities  which  may  enter  into  its  composition.  Some 
limestones  are  colored  red  from  the  presence  of  iron 
oxide.  This  red  may  vary  from  yellow  and  orange, 
through  red  to  purple.  Limestones  may  be  green 
or  greenish  from  the  presence  of  other  compounds  of 
iron,  as  in  the  case  of  glauconitic  and  chloritic  in- 
gredients. The  cuttings  of  limestones  that  come 
from  below  the  level  of  moisture  in  the  ground 
very  generally  have  a  bluish  gray  tinge,  mostly 
owing  to  the  presence  of  ferruginous  material. 
Under  the  influence  of  weathering  such  limestones 
acquire  the  familiar  yellowish,  straw  color.  Some 
limestones  are  dark  gray  or  even  black,  due  in 
some  cases  to  the  presence  of  manganese,  pyrite 
or  in  other  cases  to  the  presence  of  bituminous 
material.  As  regards  their  texture,  limestones 
vary  greatly.  Most  limestones  which  are  not  dolo- 
mitic  have  a  fine  texture  and  under  the  impact 
of  the  drill  they  break  into  fragments  that  are  more 
or  less  equal  in  diameter  in  all  directions,  though 
there  are  many  exceptions  to  this  rule.  Some  lime- 
stones are  thinly  laminated  and  break  into  flaky 
fragments.  Some  limestones  are  highly  porous  and 
others  again  are  quite  compact,  and  break  with 
smooth  and  curving  fracture  almost  like  flint.  The 
hardness  of  limestone  is  such  that  it  can  easily  be 
scratched  with  soft  iron.  Soft  limestones  can  be 
scratched  with  the  finger  nail.  The  usual  progress 
of  drilling  in  a  limestone  is  at  the  rate  of  ten  feet 
in  twelve  hours,  with  a  standard  rig,  and  a  little 
less  with  a  rotary  rig,  using  a  Hughes  bit.  It  is 
said  that  a  pure  limestone  has  a  tendency  to  wear 
the  angles  of  a  drop  bit  so  as  to  round  its  cutting 
edges. 

Drillers  usually  identify  limestones  by  mere  optic 
examination  of  the  cuttings.  In  many  cases  this  is 
unsatisfactory.  Beds  of  gypsum  and  anhydrite  have 
often  been  in  this  way  identified  as  limestones.  The 
only  reliable  and  quick  test  by  which  limestone  may 
be  identified,  is  by  the  application  of  dilute  hydro- 


Identification  of  Geological  Formations         31 

chloric  acid.  This  is  best  done  by  placing  a  few 
drops  of  the  acid  on  a  flat  glass  and  letting  a  clean 
washed  small  fragment  of  rock  fall  in  this  acid. 
Fragments  of  limestone  will  invariably  effervesce  in 
the  acid,  and  the  effervescence  is  always  prompt  and 
active.  Dolomite  effervesces  slowly.  The  finer  the 
fragments  the  mere  violent  is  always  the  efferves- 
cence. 

Varieties  of  Limestone 

When  examined  closely,  hardly  any  two  limestones 
will  be  found  alike.  With  practice,  some  of  the  va- 
rieties of  limestone  can  be  distinguished  with  the 
naked  eye,  or  at  least  with  a  hand  lens. 

Oolitic  Limestone 

Oolitic  limestone  consists  of  small,  almost  perfect- 
ly spherical  grains  of  the  size  of  fish  roe  down  to 
fine  sand.     When  the  oolitic  grains  are  of  sufficient 
size,  the  rock  is  easily  identified  with  the  naked  eye. 
Some   oolites   consist  of  grains   so   small  that  they 
can  be  identified  best  in  a  thin  section  examined  un- 
der the  microscope.     The  ooliths  are  believed  to  have 
acquired  their  round  form  by  rolling  and  by  the  ac- 
cretion of  calcareous  material  present  in  abundance 
in  the  water  where  the  rock  was  formed.     It  is  also 
believed  that  in  many  instances  the  formation  and 
growth   of  ooliths   have  been   influenced   by  micro- 
scopic organisms.     In  many  ooliths  it  can  be  made 
out  that  the  crust  of  the  grains  has  been  deposited 
around  a  small  center  of  mechanical  origin.     Oolitic 
limestones  occur  in  formations  of  all  ages.     The  pro- 
portion of  ooliths  to  the  mass  of  the  rock  varies  from 
small  to  large  fractions.     Under  the  impact  of  the 
drill,  oolitic  limestone  is  to  a  large  extent  separated 
into  the  original  oolitic  grains  and  it  is  not  infre- 
quently mistaken  for  sand.     In  this  state  horizons  of 
oolitic  material  occur  at  several  geological  horizons. 


S2  Handbook  of  Aids  to 

In  the  lower  part  of  the  Ellenburger  limestone  such 
rock  has  in  places  been  silicified,  so  that  we  find  it 
containing  layers  of  oolitic  chert.  The  ooliths  in 
this  horizon  are  relatively  large  and  almost  perfect- 
ly spherical.  They  are  also  trenchantly  marked  off 
from  the  matrix  in  which  they  are  imbedded.  In 
the  Pennsylvanian,  oolitic  limestones  are  very  rare, 
but  again  in  the  Permian,  especially  of-  the  western 
part  of  the  state,  they  are  frequent.  Some  occur  in 
the  Permian  redbeds.  In  many  of  these  Permian 
limestones  the  ooliths  are  of  small  size,  and  not  al- 
ways clearly  denned  in  outline.  In  the  Comanchean 
and  Upper  Cretaceous,  oolitic  limestones  are  prac- 
tically absent.  They  are  rarely  if  ever  found  in 
limestones  of  Tertiary  age  in  the  Southwest.  . 

Shell  Breccia 

Some  limestones  consist  of  fragments  of  shells 
which  have  been  broken  up  and  more  or  less  worn 
by  water  currents.  These  may  be  called  shell  brec- 
cias, organic  conglomerates,  or  organic  sandstones, 
according  to  the  degree  of  wear  of  the  organic  frag- 
ments from  which  they  are  made.  Many  limestones 
are  a  mixture  of  fine  calcareous  slime  material  and 
fragments  of  organic  bodies.  Such  are  most  of  the 
limestones  in  the  Pennsylvanian.  The  Anacacho 
limestone  of  the  Upper  Cretaceous  formation  is  for 
the  most  part  an  organic  sandstone  with  coarser  ]ay- 
ers  that  can  be  classed  as  shell  breccias.  Rocks  of 
this  kind  frequently  take  their  name  after  the  bulk 
cf  the  organic  materials  of  which  they  are  composed. 
Coral  limestone  consists  mainly  of  coral  fragments. 
Crinoidal  limestones  consist  of  joints  of  crinoid 
stems.  Limestone  of  this  latter  kind  is  known  in 
Texas  only  in  the  Pennsylvanian  formations. 

Chalk 

In  some  parts  of  the  sea,  in  the  past  as  well  as 
at  present,  foraminifera  have  existed  in  such  abun- 


Identification  of  Geological  Formations         33 

dance  in  the  sea  that  their  minute  shells  have  formed 
an  ooze  on  the  bottom  of  large  areas  in  the  ocean. 
When  such  deposits  are  consolidated,  they  form  the 
porous,  soft  rock  known  as  chalk.  When  exposed 
and  weathered,  chalk  is  usually  of  a  white  color. 
When  encountered  in  wells  it  is  sometimes  of  a 
bluish-gray  color,  and  so  soft  as  to  be  only  slightly 
different  from  some  marls.  Chalk  is  confined  al- 
most exclusively  to  the  Austin  formation  in  the 
upper  Cretaceous. 

Lithographic   Limestone 

This  is  characterized  by  an  exceedingly  fine  and 
uniform  texture  and  the  name  may  be  applied  to 
any  limestones  of  this  texture.  So  far  as  known, 
no  lithographic  limestone  suitable  for  use  in  the 
arts  has  been  found  in  Texas,  but  limestones  of  a 
very  fine  texture  characterize  at  least  two  geologic 
horizons.  One  of  these  horizons  is  the  upper  part 
of  the  Ellenburger  limestone;  another  is  the  Buda 
limestone  of  the  Cretaceous.  Thin  limestones  of 
like  texture  occur  also  at  certain  horizons  of  the 
Permian  above  the  Clear  Fork  formation.  These 
are  thin  and  usually  somewhat  impure.  Some  of 
them  are  characterized  by  a  peculiar  delicate  light 
blue  color. 

Dolomite 

Dolomite  and  dolomitic  limestones  are  character- 
ized by  definite  and  clear  crystallization  of  the  mass. 
On  the  application  of  acid,  effervescence  is  slow,  un- 
less the  rock  is  finely  pulverized.  The  crystals  vary 
considerably  in  size  but  are  usually  uniform  in  this 
respect  in  individual  layers  and  beds.  Usually  dolo- 
mitic limestones  are  a  little  more  resistant  to  the 
drill  than  other  limestones.  Dolomitic  limestones 
make  up  the  lower  two-thirds  of  the  Ellenburger 
formation.  Locally  there  are  some,  layers  of  dolo- 


34  Handbook  of  Aids  to 

mitic  limestone  in  the  lower  part  of  the  Comanchean. 
Dolomitic  limestone  is  rarely  found  in  the  Pennsyl- 
vanian  and  in  the  Upper  Cretaceous.  It  is  abun- 
dant in  the  Permian. 

Caliche 

Caliche  is  a  white,  porous  rock,  indurated  to  any 
extent,  or  perfectly  floury.  Such  deposits  are 
formed  by  the  deposition  of  lime  from  water  evap- 
orating from  the  surface  of  the  ground  or  from  the 
water  table  below  the  surface.  Usually  it  occurs  as 
a  part  of  the  surface  soil.  In  the  region  of  the  High 
Plains  of  northwest  Texas  it  underlies  the  yellow 
adobe  soil  at  depths  varying  from  zero  to  thirty 
or  forty  feet.  In  this  region  it  is  soft  white  rock, 
somewhat  resembling  chalk,  and  it  is  locally  known 
as  the  "rim  rock."  It  is  made  up  of  exceedingly 
fine  calcareous  crystals  of  carbonate  of  lime,  and 
effervesces  with  great  briskness  on  the  application 
of  acid. 

* 

Travertine 

Travertine  is  a  crystalline  or  occasionally  struc- 
tureless calcareous  rock  of  somewhat  more  than 
common  hardness.  It  is  sometimes  encountered  as 
fillings  in  underground  caverns  and  is  in  many 
cases  colored  red  by  the  presence  of  ferruginous  ma- 
terial. On  account  of  its  crystalline  structure  and 
its  color  it  has  in  some  cases  been  reported  by  drill- 
ers as  granite. 

Bituminous  Limestone 

This  is  a  limestone  impregnated  with  asphalt  or 
other  bituminous  material.  It  can  usually  be  recog- 
nized by  its  odor  upon  heating,  or  upon  crushing 
after  it  is  thoroughly  dried.  Limestones  so  impreg- 
nated are  usually  quite  resistant  to  the  drill.  For 


Identification  of  Geological  Formations         35 

the  most  part  they  are  thin.  The  thickest  known 
are  m  the  Permian  of  West  Texas,  near  the  middle 
of  the  Permian  section. 

GYPSUM    AND    ANHYDRITE    ROCK 

Any  large  accumulation  or  bed  of  these  minerals 
is  to  be  regarded  as  rock  and  should  be  identified 
by  drillers  in  describing  the  ground  explored. 

When  sulphate  of  lime  is  precipitated  in  the  sea, 
it  may  result  in  the  formation  of  either  of  these  two 
rocks.  Both  are  characterized  by  their  softness. 
They  can  be  scratched  with  the  finger  nail.  Neither 
of  the  two  is  attacked  by  acid,  and  this  furnishes 
a  ready  method  for  identification.  A  soft  sedimen- 
tary rock  which  resembles  limestone  in  texture  and 
which  will  not  effervesce  in  acid,  is  in  nine  cases 
out  of  ten  either  gypsum  or  anhydrite.  Anhydrite 
may  sometimes  be  known  from  its  behavior  in  drill- 
ing. When  it  is  ground  up  into  very  fine  powder 
by  the  drill,  it  is  apt  to  take  up  water  and  set  like 
plaster  of  Paris,  adhering  strongly  to  the  bit,  even 
to  such  an  extent  as  to  interfere  with  the  work. 
Anhydrite  as  well  as  gypsum  occurs  in  association 
with  the  salt  beds  of  the  salt  domes  in  the  coast  coun- 
try and  there  they  are,  like  the  salt  deposits,  limited 
in  horizontal  extent  but  often  quite  deep.  Usually 
the  anhydrite  occurs  under  the  cap  rock  and  always 
above  the  salt.  It  has  frequently  been  reported  as 
limestone  by  drillers  in  the  coastal  fields.  While 
small  crystals  and  small  bodies  of  gypsum  and  anhy- 
drite may  be  found  in  many  clays,  it  is  only  in  the 
Permian  and  perhaps  some  of  the  Comanchean  beds 
in  the  western  part  of  the  state  that  these  minerals 
exist  as  extensive  formations.  Most  of  the  sulphate 
of  lime  in  the  Permian  sea  seems  to  have  been  de- 
posited originally  in  the  form  of  anhydrite,  for  it  is 
seldom  that  any  gypsum  exists  in  these  beds  below 
300  or  400  feet,  from  the  surface.  Below  this  depth 
nearly  all  sulphate  of  lime  is  anhydrite.  Extensive 


36  Handbook  of  Aids  to 

beds  of  anhydrite  underlie  the  Panhandle  and  the 
Llano  Estacado  and  extend  nearly  to  the  central  part 
of  the  state  in  the  lower  part  of  the  Permian.  An- 
hydrite as  well  as  gypsum  has  frequently  been  re- 
ported as  limestone.  The  most  fine  grained  kinds  of 
gypsum  and  anhydrite  are  also  called  alabaster  rock. 

ROCK   SALT 

Rock  salt  is  what  the  name  implies,  salt  in  large 
masses.  In  the  salt  domes  of  the  coast  these  masses 
are  known  to  be  several  thousand  feet  in  thickness. 
They  do  not  here  reach  any  great  distance  horizon- 
tally, but  are  limited  to  the  salt  domes,  which  seldom 
extend  more  than  a  few  miles  in  any  direction.  The 
only  formation  which  contains  rock  salt  in  exten- 
sive horizontal  beds,  is  the  upper  part  of  our  Per- 
mian in  the  Panhandle  and  in  the  Llano  Estacado 
regions.  Here  we  find  salt  beds  from  a  few  up  to 
300  feet  in  thickness,  evidently  extending  scores  of 
miles  underground.  They  represent  evaporated  brine 
from  embayments  in  the  sea  in  which  the  Upper 
Permian  was  laid  down.  At  times  in  this  sea,  salt 
was  evidently  accumulating  at  the  same  time  with 
fine  sand  and  silt,  and  the  result  is  that  we  find 
beds  which  consist  of  half  salt  and  half  silt.  In 
the  Spur  well,  such  beds  were  some  scores  of  feet 
in  thickness. 

In  drilling  through  rock  salt,  especially  with  a 
rotary,  it  sometimes  happens  that  the  ground-up 
rock  is  entirely  dissolved  by  the  water  used.  In 
such  cases  it  has  sometimes  puzzled  inexperienced 
men  that  there  are  no  "returns."  The  rock  is 
all  dissolved  and  there  are  no  cuttings  from  the 
ground  penetrated. 

FLINT    AND    CHERT 

All  calcareous  muds  that  have  been  consolidated 
into  limestone  contained  originally  some  siliceous 


Identification  of  Geological  Formations         37 

material*  Under  the  influence  of  moisture  present 
in  the  ground,  different  minerals  have  a  tendency 
in  all  rocks  to  segregate.  Each  mineral  tends  to 
form  separate  bodies  of  its  own  kind.  By  a  process 
somewhat  resembling  the  growth  of  crystals,  there 
are  thus  formed  frequently  in  sedimentary  rocks 
bodies  of  materials  of  different  composition  from 
the  mass  of  the  rock  itself.  These  bodies  are  called 
concretions.  Their  shape  usually  is  more  or  less 
determined  by  the  structure  of  the  rocks.  In  bedded 
rocks  there  is  a  tendency  for  concretions  to  be  flat 
or  loaf-like.  When  the  siliceous  materials  present 
in  limestones  have  gathered  in  concretions,  we  call 
these  bodies  flint  or  chert.  Concretions  of  this  kind 
have  formed  in  many  limestones  and  are  frequently 
characteristic  of  certain  layers  in  these  limestones. 
Flint  has  a  hardness  a  little  in  excess  of  the  steel  in 
a  file.  It  will  cut  glass  and  indent  some  steel.  It 
may  have  any  color,  but  is  usually  gray  or  grayish- 
white  when  found  below  the  surface.  It  can  usually 
be  known  from  its  fine  texture,  and  from  its  tend- 
ency to  split  into  very  sharply  angular  fragments. 
When  viewed  with  a  magnifying  glass  the  chert  is 
translucent  along  thin  edges.  As  quartz  is  not  af- 
fected by  acids,  flint  and  chert  can  readily  be  told 
from  limestone  when  present  in  cuttings.  With 
practice  it  is  readily  recognized  without  any  test. 
Flint  is  frequent  in  the  Pennsylvanian  limestones 
and  in  parts  of  the  Ellenburger  limestone  of  the 
Ordovician.  It  is  infrequent  in  our  Permian  lime- 
stones, but  quite  abundant  in  the  Edwards  limestone 
of  the  Comanchean  rocks.  In  the  Upper  Cretaceous, 
flint  is  rare,  and  it  is  almost  unknown  in  our  Ter- 
tiary sediments. 

CLAY-IRON-STONE 

Just  as  silica  is  present  in  limestone,  so  we  have 
carbonate  of  iron  present  in  shales.  In  some  shales 
this  carbonate  of  iron  is  distributed  throughout  the 


38  Handbook  of  Aids  to 

deposit,  while  in  other  cases  it  has  formed  concre- 
tions and  gathered  together  into  nodules  and  loaf- 
like  bodies  ranging  in  size  from  a  few  inches  to  many 
feet  in  diameter.  Occasionally  an  accumulation  of 
carbonate  of  iron  forms  separate  layers,  which,  no 
doubt,  were  laid  down  originally  in  their  present 
condition.  When  heated  in  a  flame,  carbonate  of 
iron  is  reduced  in  part  to  metallic  iron  and  becomes 
magnetic,  so  that  if  a  magnet  is  passed  through 
the  cuttings  the  roasted  fragments  of  clay-iron-stone 
will  be  picked  up.  It  is  also  attracted  by  acid  but 
the  effervescence  is  much  more  slow  than  it  is  in  the 
case  of  limestone  and  dolomite.  Clay-iron-stone  is 
of  a  fine  texture  and  gray  in  color.  It  is  quite  hard 
and  very  tough  and  resistant  to  the  drill.  Most 
clay-iron-stone  concretions  are  traversed  by  thin 
veins  of  calcite  which  is  frequently  colored  brown 
from  the  presence  of  iron. 

Concretions  of  this  kind  are  frequent  in  all  dark 
clays  and  shales.  Drillers  often  record  them  as 
boulders.  They  are,  of  course,  not  true  boulders, 
since  they  have  grown  into  shape  in  the  place  where 
they  are  found.  Concretions  of  carbonate  of  iron 
occur  in  a  few  horizons  of  the  dark  shales  of  the 
Pennsylvanian.  They  are  frequent  and  often  of 
large  size  in  the  Taylor  Marl  and  in  the  Navarro 
formation  of  the  Upper  Cretaceous,  and  they  are 
quite  common  in  the  clays  and  marls  of  the  Tertiary. 
The  true  nature  of  these  formations  can  easily  be 
ascertained  by  the  fire  test,  as  already  mentioned. 


Identification  of  Geological  Formations         39 

THE  PRINCIPAL  FORMATIONS  IN  TEXAS 
INTRODUCTORY 

What  is  known  among  geologists  as  "formations" 
are  extensive  sheets  of  more  or  less  similar  rock 
material  that  have  been  deposited  under  somewhat 
similar  conditions,  over  large  areas.  Just  how 
much  is  to  be  called  one  formation,  is  to  a  great 
extent  dependent  on  the  judgment  of  those  who  de- 
scribe the  formations.  In  cases  where  the  forma- 
tions are  clearly  marked  by  difference  in  the  nature 
of  the  materials  of  which  they  consist,  or  by  differ- 
ences in  the  fossils  they  contain,  there  is  seldom  any 
disagreement  as  to  their  limits.  The  horizontal  ex- 
tent of  formations  is  very  variable,  ranging  from 
a  few  miles  to  several  hundred  miles.  As  the  ex- 
istence of  water,  oil,  and  other  minerals,  which  are 
sought  by  drilling,  can  to  some  extent  be  foretold 
from  the  nature  and  position  of  the  formation  in 
which  they  occur,  it  is  of  much  importance  to  the 
practical  driller  to  be  able  to  distinguish  the  dif- 
ferent formations  he  penetrates.  This  is  usually 
done  by  examining  the  cuttings  as  the  drill  descends. 
Such  identification  of  formations  requires  a  most  ex- 
tensive as  well  as  intimate  knowledge  of  the  rock  of 
which  they  consist.  In  making  them,  thfe  driller  has 
an  advantage  over  the  geologist  in  that  he  has  op- 
portunities to  make  frequent  examination  of  the  ma- 
terials through  which  he  drills.  On  the  other  hand, 
the  geologist  usually  has  the  advantage  in  possess- 
ing extensive  knowledge  of  the  formations  from  ex- 
amining them  in  exposures  at  the  surface,  and  in 
widely  separated  places.  This  enables  him  to  better 
interpret  his  observations  on  the  material  brought 
up  by  the  drill.  It  is  higTily  desirable  that  the  ef- 
forts of  drillers  and  of  geologists  should  be  com- 
bined in  securing  exact  knowledge  of  our  formations 


40  Handbook  of  Aids  to 

as  they  exist  underground.  These  notes  are  writ- 
ten in  the  hope  that  they  may  be  of  some  aid  in  this 
direction.  They  are  brief  descriptions  of  the  prin- 
cipal formations  which  are  encountered  by  drillers 
in  the  southwest. 

In  these  notes,  the  formations  have  been  described 
in  order  from  above  downward.  This  is  the  order  in 
which  the  driller  encounters  them.  It  is  hardly  nec- 
essary to  call  the  attention  of  the  reader  to  the  fact 
that  all  of  these  formations,  from  the  top  to  bottom, 
are  not  found  at  any  one  place.  Some  are  always 
absent. 

The  latest  formation  overlies  the  older  rocks  al- 
most everywhere,  especially  in  stream  valleys  and 
on  the  coast.  There  is  a  great  accumulation  of  late 
deposits  along  the  Gulf  Coast.  Formations  of  ear- 
lier age  than  these  are  encountered,  superficially,  in 
succession  further  inland  away  from  the  Gulf,  until 
we  come  to  the  Central  Mineral  Region  in  Llano 
and  surrounding  counties.  Here  we  encounter  some 
of  the  very  oldest  rocks.  From  this  point  north- 
westward we  find  again  successively  younger  beds 
out  to  the  northwestern  part  of  the  state,  where 
recent  deposits  overlie  the  older  ones  on  the  High 
Plains.  In  the  region  west  of  the  Pecos  River,  the 
country  has  been  broken  up  into  parts  of  relatively 
small  size,  and  we  find  all  the  formations  outcrop- 
ping at  the  surface  within  quite  short  distances  of 
each  other* 

Everywhere  it  will  be  found  that  some  of  the 
formations  described  are  missing,  in  local  sections. 
Gaps  of  this  kind  are  in  many  cases  due  to  earlier 
periods  of  erosion,  by  which  some  formations  were 
removed  before  later  ones  were  deposited.  In  such 
case  the  beds  below  and  above  the  missing  member 
or  members,  are  said  to  be  separated  by  unconformi- 
ties. 

A  peculiar  condition  exists  on  the  coast.  By  far 
the  greatest  part  of  all  borings  on  the  coast  have 


Identification  of  Geological  Formations         41 

been  made  in  the  so  called  "salt  dome  oil  fields'* 
in  that  region.  More  than  a  score  and  a  half  of 
salt  dome  structures  have  been  found  on  the  coastal 
plain,  mostly  within  a  hundred  miles  of  the  Gulf 
Coast.  These  structures  are  dome-like  and  cover 
relatively  small  areas  from  one  to  three  or  four 
miles  in  diameter.  Their  origin  is  not  xet  fully  un- 
derstood. They  may  be  said  to  resemble  plug-like  up- 
lifts of  ground,  where  beds  lying  elsewhere  at  a  depth 
of  3000  to  4000  feet  have  been  pushed  up  to  with- 
in 1000  feet  of  the  surface  or  even  up  to  the  surface- 
Where  most  of  the  drilling  in  the  Gulf  Coast  oil  fields 
has  ben  done,  the  original  position  of  the  formations 
is  therefore  quite  destroyed  and  the  descriptions  and 
measurements  which  are  given  in  the  following  note5. 
on  the  Pleistocene  and  Tertiary  do  not  apply  to  the 
positions  which  these  formations  have  in  the  salt 
dome  oil  fields,  where  most  drillers  have  had  their 
experience.  They  apply  only  to  the  positions  that 
these  rocks  occupy  in  the  wide  tracts  which  separate 
these  domes  from  one  another. 

PLEISTOCENE  AND   RECENT   FORMATIONS 

Our  latest  deposits  are  made  up  of  the  soil,  and 
of  alluvial  sediments  of  sijt,  sand  and  gravel  along 
streams  everywhere.  On  the  Gulf  border,  littoral 
sediments,  such  as  clays,  sands,  and  gravels,  and 
mixtures  of  such  materials  of  varied  color,  extend 
to  depths  of  from  a  few  to  1600  feet.  These  thin 
rapidly  inland,  and  disappear  entirely  in  from  fifty 
to  ninety  miles  from  the  Gulf.  Logs  of  wood,  shells 
like  those  from  the  present  Gulf  waters  and  bones 
of  larger  animals,  such  as  elephants,  horses,  the 
bison,  and  the  deer,  are  occasionally  to  be  found. 
In  a  recent  boring  in  Kleberg  County,  the  minute 
oval  and  spirally  marked  spore-fruits  of  Chara  were 
found  in  cuttings  coming  from  800  feet  down,  in 
these  beds.  The  High  Plains  of  the  west  are  also 
covered  with  loam  sand  and  gravel  of  this  age. 


42  Handbook  of  Aids  to 

Almost  everywhere  the  Pleistocene  and  recent  form- 
ations are  entirely  unconsolidated. 

THE  TERTIARY  FORMATIONS 

The  tertiary  in  Texas  measures  about  5000  feet 
in  greatest  thickness.  It  underlies  the  Pleistocene 
of  the  Gulf  Coast  and  extends  in  a  belt  diagonally 
from  Louisiana  to  Mexico.  This  belt  is  about  120 
miles  wide  on  the  Rio  Grande,  70  miles  wide  in  the 
middle  of  its  course,  and  180  miles  wide  in  the 
east  part  of  the  state.  The  oldest  formations  of  the 
Tertiary  outcrop  farthest  inland  and  the  youngest 
formations  appear  nearest  the  Gulf.  The  entire 
Tertiary  dips  toward  the  Gulf  at  an  average  rate  of 
from  twenty-five  to  seventy  feet  per  mile. 

The  Tertiary  can  be  divided  into  three  parts:  the 
upper,  the  middle,  and  the  lower. 

THE  UPPER  TERTIARY 

On  the  coastal  plain,  the  Upper  Tertiary  consists 
of  deposits  much  like  those  of  the  Pleistocene,  such 
as  clays  of  various  shades  of  color,  sand,  and  gravel. 
The  clay  is  often  reported  as  gumbo  by  drillers.  In 
cuttings  from  this  part  of  the  Tertiary,  fragments 
of  bone  and  of  silicified  and  lignitized  wood  are 
sometimes  found.  Small  shells  of  fresh  water  mol- 
luscs (such  as  clams  and  snails)  and  fragments  of 
larger  molluscs  are  somewhat  common,  in  cuttings 
from  these  beds  on  the  coast.  Some  of  the  sands 
are  coarse,  some  fine  in  texture.  In  the  clays,  small 
calcareous  concretions  are  frequently  found.  Inland, 
the  upper  Tertiary  deposits  are  mostly  old  alluvial 
accumulations  yet  but  little  known.  Their  thick- 
ness is  very  variable,  at  different  points,  but  prob- 
ably nowhere  exceeds  four  hundred  feet  in  the  in- 
terior of  the  state. 


Identification  of  Geological  Formations         43 

THE   MIDDLE   TERTIARY 

On  the  Gulf  Coastal  Plain,  the  Middle  Tertiary 
consists  of  two  divisions.  The  Dewitt  formation, 
from  0  to  800  feet  thick,  lies  uppermost.  It  con- 
sists mostly  of  slightly  indurated  sand  with  some 
streaks  of  clay.  These  sands  and  clays  often  con- 
tain bones  and  ancient  oyster  banks.  The  lower 
part  of  the  Middle  Tertiary,  known  as  the  Fleming 
Clay,  is  from  200  to  400  feet  thick.  It  is  very  gen- 
erally reported  by  drillers  as  "gumbo,"  as  it  is  fine- 
textured  and  sticky.  It  is  noted  for  containing 
many  small  nodules  or  concretions  of  calcareous  ma- 
terial. 

In  the  High  Plains  of  the  Panhandle,  and  the 
Llano  Estacado,  the  Middle  Tertiary  is  •  represented 
by  fairly  loose  sands  and  clays,  mostly  less  than  100 
feet  thick,  and  from  light  gray  and  greenish-gray 
to  brown  in  color.  Fragments  of  bones  of  mammals 
are  quite  frequent. 

The  Catahoula  sandstone  may  be  said  to  be  the 
lowermost  formation  of  the  Middle  Tertiary,  meas- 
uring from  500  to  800  feet.  It  is  coarsest  below  and 
contains  some  clay  above.  The  coarser  sand  of  this 
formation,  especially  near  its  base,  is  sometimes 
characterized  as  "rice  sand."  This  is  because  many 
of  the  larger  sand  grains  are  oval.  They  consist  of 
a  slightly  translucent  quartz  resembling  agate  or 
chalcedony.  The  sands  of  this  formation  are  usu- 
ally soft  but  locally  they  may  be  hard.  Fossil  wood 
is  quite  common  in  this  formation. 

THE    LOWER   TERTIARY 

The  Lower  Tertiary  occurs  only  on  the  inner  Gulf 
Coast  Plains,  where  it  is  represented  by  no  less  than 
seven  different  formations. 

Under  the  Catahoula  is  the  Jackson  formation 
in  iSabine,  San  Augustine,  and  Angelina  counties. 
It  consists  of  some  250  feet,  or  less,  of  marly  clay 


44  Handbook  of  Aids  to 

with  sea  shells  and  large  calcareous  concretions, 
usually  known  as  "boulders."  It  can  often  be  iden- 
tified under  the  microscope  by  the  presence  of  many 
angular  grains  of  volcanic  dust. 

Next  in  downward  order  is  the  Yegua  clay,  rang- 
ing from  375  to  750  feet  in  thickness.  The  clay  is 
greenish-gray  and  dark  gray.  It  contains  lignite 
in  places  and  is  characterized  by  concretions  and 
crystals  of  gypsum,  and  by  frequent  minute  siliceous 
tests  of  diatoms,  some  of  which  are  of  relatively 
large  size  and  spherical  in  form. 

The  Yegua  is  underlain  by  the  Cook  Mountain 
formation,  which  consists  of  sand  and  clay.  Both 
the  sand  and  the  clay  often  contain  green  grains  of 
glauconite,  frequent  fossils,  many  concretions,  and 
often  continuous  layers  of  clay-iron-stone.  Thick- 
ness, 400  feet. 

The  underlying  Mount  Selman  formation  is  some 
350  feet  thick  and  consists  of  materials  somewhat 
like  those  of  the  Cook  Mountain,  but  in  part  red, 
and  with  more  lignite. 

The  Wilcox  formation  measures  some  1000  feet 
in  thickness.  This  formation  consists  of  marly 
clays  and  beds  of  sand  some  of  which  yield  much 
water.  There  is  also  much  lignite  and  some  glau- 
conite. Fossil  shells  and  plants  are  frequent  .  in 
some  of  the  marl. 

The  Midway  is  the  lowest  of  the  Tertiary  forma- 
tions. From  Hopkins  to  Falls  County  this  forma- 
tion contains  considerable  white  limestone,  which  is 
an  indurated  shell  breccia  or  a  sandstone  made  up 
of  mostly  worn  organic  grains  mingled  with  some 
foraminifera,  and  cemented  by  a  calcareous  matrix. 
The  greater  part  of  the  formation  which  is  some  400 
feet  in  thickness  consists  of  thin-bedded  marly  clays, 
with  imbedded  small  fossils  and  black  shreds  of 
vegetation.  Minute  tests  of  foraminifera  are  quite 
common. 


Identification  of  Geological  Formations        45 
THE  UPPER  CRETACEOUS  FORMATIONS 

The  Upper  Cretaceous  formations  are  a  succession 
of  clay,  shale,  or  marl,  chalky  flags,  and  sandstone. 
They  outcrop  in  a  belt  extending  from  the  northeast 
corner  of  the  state  westward  to  beyond  Dallas,  from 
there  south  to  San  Antonio,  and  thence  west  to  Del 
Rio.  The  thickness  of  the  entire  Cretaceous  is  near 
5000  feet.  In  east  and  central  Texas,  the  formations 
of  this  age  dip  toward  the  Gulf  at  a  varying  rate 
from  a  few  to  some  eighty  feet  to  the  mile,  and 
south  and  east  from  the  indicated  belt  they  pass 
under  the  deposits  of  Tertiary  age,  going  deeper  and 
deeper  as  we  approach  the  Gulf.  Here  they  may  lie 
more  than  5000  feet  below  the  surface.  West  of  the 
Front  Range,  the  Cretaceous  occurs  north  of  and 
around  the  Chisos  Mountains,  between  the  Chinati, 
the  Eagle,  and  the  Quitman  mountains,  east  of  Davis 
Mountains,  and  in  some  other  less  extensive  areas. 

The  latest  of  the  Upper  Cretaceous  is  known  as 
the  Tornillo  beds  and  these  consist  of  vari-colored, 
red,  yellow,  and  greenish-gray  and  dark  clays.  These 
occur  in  a  belt  surrounding  the  Chisos  Mountains, 
and  are  of  little  interest  to  drillers. 

The  Tornillo  clays  are  underlain  by  some  coal- 
bearing  beds,  called  the  San  Carlos  beds,  in  Presi- 
dio County  and  the  Rattlesnake  beds  in  Brewster 
County.  These  may  be  the  equivalents  of  the  coal- 
bearing  beds  at  Eagle  Pass.  So  far  these  forma- 
tions have  been  but  little  drilled.  With  the  over- 
lying Escondido  beds  in  Maverick  County,  they  must 
have  a  thickness  of  at  least  a  thousand  feet  in  the 
southeast  part  of  Maverick  County.  The  clays  are 
mostly  dark,  tough,  and  gumbo-like.  These  alter- 
nate with  soft  sandstones  from  10  to  100  feet  in 
thickness.  These  are  frequently  strongly  cemented 
with  calcareous  material  and  again  quite  soft,  al- 
most loose  sands.  There  are  also  layers  of  oyster 
shell  beds. 


46  Handbook  of  Aids  to 

In  the  Anacacho  Mountains  and  east  into  Medina 
County  the  Eagle  Pass  coal-bearing  formation  seems 
to  be  replaced  by  the  Anacacho  limestone.  This 
formation  is.  some  400  feet  thick  in  the  Anacacho 
Mountains,  but  thins  gradually  to  the  east.  It  con- 
sists of  a  white  limestone  composed  of  some  oolitic 
material,  much  worn  calcareous  beach  sand  of  or- 
ganic origin,  and  some  fine  shell  gravel.  For  the 
greater  part,  the  rock  is  strongly  cemented  by  in- 
filtrated and  other  fine-textured  calcareous  material. 
It  is  the  thickest  oolitic  limestone  among  our  Cre- 
taceous formations.  The  somewhat  open  textured 
beach  gravel  in  the  lower  part  of  the  formation  in 
Uvalde  County* contains  the  Uvalcle  asphalt. 

Below  the  coal-bearing  beds  of  the  Upper  Creta- 
ceous we  have  in  north  Texas  the  Navarro  beds, 
which  are  regarded  as  measuring  some  300  or  400 
feet  in  thickness.  These  are  mostly  gray  marls  and 
sands  which  nearly  everywhere  contain  some  glau- 
conite,  by  which  they  are  usually  recognized. 

In  the  north,  the  Navarro  beds  are  not  easily  to  be 
separated  from  the  underlying  Taylor  marl  by  the 
examination  of  cuttings.  This  formation  is  one  of 
the  well  known  "gumbos"  of  the  upper  coastal 
plains.  It  is  an  easy  drilling  formation,  through 
the  700  to  800  feet  of  which  rotaries  have  been 
plunged  in  some  cases  in  less  than  three  days  in 
the  Corsicana,  the  Thrall,  and  the  Caddo  fields.  In 
some  places  small  fine-textured  sandy  layers  occur 
in  the  Taylor  marl  and  such  sands  are  usually  glau- 
conitic,  as  is  also  frequently  the  marl  itself.  Occa- 
sionally small  teeth  of  fish  come  up  with  the  returns 
from  the  upper  third  of  this  formation  and  through-- 
out  nearly  the  whole  occur  prisms  of  calcareous  ma- 
terial from  shells  of  Inocerami.  The  marl  is  almost 
invariably  more  or  less  bituminous. 

The  Austin  Chalk,  from  200  to  300  feet  in  thick- 
ness, underlies  the  Taylor  marl.  This  is  usually 
easily  recognized  by  its  texture,  its  light  color  and 


Identification  of  Geological  Formations         47 

its  softness.  In  some  places  it  is  bluish  gray.  This 
can  also  be  recognized  by  the  abundance  of  small 
formaminifera  which  it  yields  on  trituration  and 
washing.  The  prisms  of  shells  of  Inocerami  are  al- 
ways present  in  the  cuttings,  and  frequently  also 
larger  pieces  of 'these  shells.  The  Austin  Chalk  is 
a  quite  uniformly  developed  formation  wherever 
it  occurs  in  the  southeast  two-thirds  of  the  state. 
West  of  the  Pecos  it  is  less  easily  distinguished 
from  the  overlying  Taylor  and  the  underlying  Eagle 
Ford. 

The  Eagle  Ford  formation  is  quite  unequally  de- 
veloped in  different  parts  of  the  state.  To  the  north 
and  northeast  it  merges  downward  into  the  Wood- 
bine sands  and  consists,  like  this,  of  marls,  clays  and 
sands.  Here  it  is  several  hundred  feet  thick. 
Southward  it  gradually  becomes  thinner  until  at 
Austin  it  is  less  than  50  feet.  Westward  from  San 
Antonio,  it  again  increases  in  thickness.  Following 
it  beyond  the  Pecos  River,  we  find  that  it  measures 
some  600  feet.  From  Austin,  south  and  west,  it 
becomes  more  and  more  calcareous  and  west  of  Dev- 
il's River  it  is  mostly  thin-bedded  limestone.  This 
formation  is  everywhere  more  bituminous  than  other 
parts  of  the  Cretaceous  and  this  feature  is  most 
marked  where  it  has  the  least  thickness,  as  from 
Waco  to  San  Antonio  and  from  San  Antonio  to  Del 
Rio.  It  nearly  everywhere  gives  showings  of  oil. 
Within  the  same  limits  it  consists  in  part  of  black 
marly  shale  containing  in  places  as  much  as  ten  or 
even  twenty  per  cent  of  oil  or  other  volatile  bitu- 
mens. This  black  shale  and  also  the  more  calcareous 
layers  which  approach  limestone  or  sandstone,  can 
usually  be  known  in  cuttings  by  the  occurrence  of 
fish  scales  or  other  fish  remains  on  flat  cleavage 
fractures.  Such  scales  are  less  frequent  in  the  for- 
mation in  the  north  and  in  the  west,  but  they  can 
generally  be  found  in  cuttings  from  these  localities 
also,  if  persistently  looked  for.  In  Bexar  County 


48  Handbook  of  Aids  to 

many  drillers  record  this  formation  as  "lignite"  in 
their  logs. 

The  Eagle  Ford  is  underlain  from  Waco  north- 
ward by  the  Woodbine  sands  and  clays.  These  are 
not  always  readily  to  be  separated  in  cuttings  from 
the  Eagle  Ford.  In  the  Woodbine,  sands  are  more 
prevalent  than  in  the  overlying  formation,  and  this 
formation  generally  shows  some  traces  of  plants, 
such  as  shreads  of  leaves  and  pieces  of  wood,  while 
it  rarely  has  any  fish  remains.  Pyrite  is  common 
in  places  near  the  base  of  this  formation. 

THE  LOWER  OR  COMANCHEAN   CRETACEOUS  FORMA- 
TIONS 

The  Comanchean  Cretaceous  (Lower  Cretaceous) 
is  essentially  limestone  with  some  marly  clay  am 
sandstone.  It  is  one  of  the  most  extensive  series  of 
rocks  in  Texas,  covering  a  belt  of  country  from 
Gainesville  to  north  of  San  Antonio  and  from  there 
westward  over  the  entire  Edwards  Plateau  to  the 
Front  Range  and  into  New  Mexico.  Smaller,  but 
quite  extensive  areas  occur  west  of  the  Front  Range. 
Its  thickness  ranges  from  zero  to  at  least  1500  feet 
in  the  Big  Bend  Country.  It  has  been  drilled  quite 
extensively,  mostly  for  water. 

Different  ones  of  the  Comanchean  formations  can 
be  separately  identified. 

South  of  Waco  and  westward  past  San  Antonio 
and  out  to  Brewster  County,  the  Buda  limestone  un- 
derlies the  Upper  Cretaceous.  This  is  a  pure  white 
and  almost  compact  limestone,  seldom  less  than 
thirty  or  more  than  seventy  feet  in  thickness.  It 
is  sharply  marked  off  from  the  dark  Eagle  Ford 
clay  above  and  from  the  soft  Del  Rio  clay  below. 
No  other  formation  in  Texas  compares  with  the 
Buda  in  the  combination  of  its  white  color  and  fine- 
ness of  texture. 
.  The  Del  Rio  clay,  which  underlies  the  Buda,  has 


Identification  of  Geological  Formations         49 

a  great  capacity  for  caving.  In  places  it  is  absent 
and,  where  present,  it  varies  in  thickness  from  a 
few  feet  to  two  hundred  feet.  This  formation  can 
almost  everywhere  be  identified  by  a  little  "ram's 
horn"  shell,  Exogyra  arietina,  and  another  equally 
common  shell,  which  looks  like  a  very  short  string 
of  small  beads,  Nodosaria  texana.  Both  can  gen- 
erally be  washed  from  the  returns,  from  this  clay. 

In  the  central  part  of  the  state  the  Georgetown, 
which  underlies  the  Del  Rio,  consists  of  frequent 
alternations  of  limestone  and  marls  usually  full  of 
shells.  To  the  southwest  this  is  replaced  by  some 
very  compact  limestone. 

Under  the  Georgetown  follows  the  thickest  and 
most  extensively  developed  member  of  the  Coman- 
chean,  the  Edwards  limestone.  This  measures  frori 
fifty  to  four  hundred  feet  in  thickness.  It  is  quite 
uniform  in  character,  partly  of  coarse  crystalline 
texture,  in  some  places  cavernous,  and  almost  every- 
where it  contains  a  few  persistent  layers  of  gray 
flint. 

Below  the  Edwards  there  are  again  some  changes 
to  marly  material  and  often  some  strata  containing 
fine  sand,  the  Paluxy  sands,  which  are  generally 
water-bearing,  and  are  oil-bearing  near  South 
Bosque.  At  this  depth  the  Glen  Rose  formation  be- 
gins. This  can  usually  be  identified  from  its  cut- 
tings by  the  appearance  in  them  of  some  small  cir- 
cular, concavo-convex,  dark,  disk-shaped  fossils, 
Orbitulina  texana.  The  limestones  below  the  Ed- 
wards are  quite  generally  noticeably  finely  dark- 
speckled,  and  are  almost  invariably  darker  than  the 
overlying  rock.  They  measure,  with  the  interbed- 
ded  marls  and  sands,  from  three  hundred  to  nearly 
a  thousand  feet  in  thickness. 

The  basal  deposits  of  the  Comanchean  consist  of 
mostly  blue  and  partly  red  marls  and  white,  gray, 
or  yellowish  sand,  varying  from  packsand  to  gravel. 
They  are  mostly  quite  soft,  but  are  in  a  few  places 


50  Handbook  of  Aids  to 

very  hard,  practically  quartzite.  This  sand,  which 
is  the  Trinity  sand,  makes  the  great  water-bearing 
formation  in  northern,  and  parts  of  central  Texas. 

THE  JURASSIC  FORMATIONS 

Deposits  of  this  age,  consisting  chiefly  of  lime- 
stone with,  some  conglomerates  and  dark  shale,  are 
known  only  in  the  Malone  Mountains  and  are  of  no 
general  importance  to  drillers  in  Texas. 

THE  TRIASSIC  FORMATIONS 

This  consists  largely  of  red  clays  and  sand,  with 
which  are  also  some  gray  sands  and  some  conglom- 
erates. 

The  Triassic  underlies  the  greater  part  of  the 
Panhandle  and  the  Llano  Estacado,  where  it  over- 
lies the  Permian  and  extends  in  detached  outliers 
as  far  east  as  to  Glasscock  County  at  the  south  and 
beyond  Dickens  and  Amarillo  to  the  north.  The 
sands  and  conglomerates  of  these  beds  usually 
have  good  water.  These  sands  are  mostly  highly 
micaceous.  The  conglomerates  in  the  lower  part 
are  characterized  by  well  rounded  yellow  or  white 
quartz  pebbles.  All  parts  of  the  formation  are  apt 
to  contain  logs  of  trees  changed  to  lignite  or  in  a 
silicified  condition.  Some  gypsum  is  also  found, 
and  black  manganese  oxide  is  not  infrequent.  The 
Triassic  is  not  known  to  have  any  limestones. 

THE  PERMIAN  FORMATIONS 

\ 

The  greatest  known  thickness  of  the  Permian  is  in 
the  Glass  Mountains,  where  it -measures  some  9000 
feet  and  consists  largely  of  limestone,  in  part  dolo- 
mitic.  Permian  of  the  same  general  character  oc- 
curs also  in  the  Guadalupe  Mountains,  north  and 
east  of  the  Chinati  Mountains,  and  probably  in  some 
other  localities  west  of  the  Pecos. 


Identification  of  Geological  Formations         51 

The  Permian  beds  which  are  apt  to  be  of  most 
interest  to  drillers  occur  on  the  Plains.  This  form- 
ation extends  west  from  the  principal  Pennsylvanian 
area  to  the  Pecos  River  and  to  the  east  escarpment 
of  the  Panhandle  High  Plains  and  the  Llano  Esta- 
cado.  West  from  this  escarpment  it  is  overlain  by 
the  Triassic  and  other  later  formations,  rising  again 
in  New  Mexico.  How  far  south  it  underlies  the  Co- 
manchean  north  of  the  Rio  Grande  and  east  of  the 
Pecos  River  is  not  known.  It  has  been  found  in  a 
boring  at  Sheffild.  The  upper  1500  feet  of  this  part 
of  the  Permian  consists  largely  of  red  marls  and  clays 
commonly  known  as  the  "red  beds"  which  also  con- 
tain beds  of  anhydrite  and  gypsum  and  quite  fre- 
quent beds  of  salt,  some  of  the  latter  measuring  as 
much  as  a  hundred  feet  in  thickness.  These  beds 
are  mostly  eroded  away  east  of  the  line  joining 
Ballinger,  Abilene,  and  Petrolia.  The  lower  part 
of  the  Permian,  which  underlies  the  "red  beds"  in 
this  region  and  which  appears  on  the  surface  east 
of  the  line  indicated  as  far  as  to  the  Pennsylvanian 
area,  consists  of  mostly  blue,  in  part  reddish,  and 
purplish  shales,  clays,  and  marls;  and  there  are 
gray,  red  and  white  sandstones,  which  are  nearly 
always  of  fine  texture  and  only  slightly  micaceous. 
Even  this  part  of  the  Permian  in  places  contains 
some  gypsum  and  salt.  The  limestones  are  mostly 
thin  and  in  many  cases  bituminous  and  fine-grained, 
and  some  are  oolitic.  There  are  also  dolomites,  which 
are  mostly  also  thin,  and  may  show  bituminous  ma- 
terial. Nearly  all  water  found  in  the  Permian  of 
the  plains  is  salt  or  even  briny.  Some  of  the  shales 
and  marls  in  the  Permian  of  the  Plains  have  a  pe- 
culiar delicate  dove-blue  color. 

THE  PENNSYLVANIAN  FORMATIONS 

The  Pennsylvanian  underlies  extensive  parts  of 
Texas.  It  is  exposed  in  the  north  central  part  of  the 
state  which  may  be  roughly  encompassed  by  a  peri- 


52  Handbook  of  Aids  to 

phery  extended  from  Montague  to  San  Saba,  from 
there  to  near  Paint  Rock,  and  then  to  Henrietta. 
West  from  this  belt  it  dips  under  the  Permian  and 
has  been  entered  in  deep  borings  at  San  Angelo  and 
at  Spur.  West  of  the  state  it  again  comes  up  to 
the  surface  east  of  the  Front  Range  in  New  Mexico 
as  well  as  in  Texas,  so  that  it  no  doubt  underlies, 
at  depths  from  3000  to  5000  feet,  the  entire  Pan- 
handle and  the  Llano  Estacado.  The  Pennsylvanian 
no  doubt  underlies  the  Comanchean  Plateau  from 
the  latitude  of  San  Angelo  to  the  Mexican  Border. 
Eastward  from  the  line  joining  San  Saba  and  Mon- 
tague, the  Pennsylvanian  underlies  the  Comanchean 
at  increasing  depths  to  quite  near  the  Balcones  Es- 
carpment. Whether  it  also  underlies  the  Coastal 
Plain  east  of  the  Balcones  Escarpment  can  not  yet 
be  regarded  as  proved,  though  it  seems  likely.  In 
this  direction  it  is  covered  first  by  the  Cretaceous 
and  farther  out  east  and  south  by  the  Tertiary  and 
may  be  looked  for  at  depths  from  5000  to  6000  feet 
or  still  deeper  near  the  coast.  West  of  the  Pecos, 
the  Pennsylvanian  no  doubt  underlies  most  of  the 
region,  though  at  greatly  varying  depths.  It  out- 
crops in  the  Franklin,  the  Hueco,  the  Chinati  and 
in  other  mountains  and  in  the  Marathon  and  the 
Solitario  uplifts  and  is  elsewhere  mostly  covered  by 
later  deposits,  in  places  at  depths  of  evidently  many 
thousand  feet. 

The  formations  making  the  Pennsylvanian  series 
measure  in  different  parts  of  the  state  from  noth- 
ing, where  eroded  away,  to  five  or  six  thousand  feet, 
as  in  the  Marathon  region.  They  consist  of  shales, 
sandstones,  and  limestones,  in  part  in  frequent  al- 
ternations. The  shales  and  clays  are  mostly  en- 
tirely free  from  calcareous  material,  by  which  fact 
they  can  usually  be  recognized  from  Cretaceous 
clays.  In  the  lower  part  of  the  series  they  are  most- 
ly dark  or  bluish  gray.  Higher  up  they  are  partly 
reddish,  brownish,  or  purplish,  which  colors  alternate 


Identification  of  Geological  Formations        53 

with  gray,  bluish  and  dark  colors.  In  the  central 
part  of  the  state  they  contain  some  small  seams  of 
coal.  Many  of  the  clays  are  fine-textured,  and  show 
slickensides  or  jointing.  The  darker  shales  general- 
ly contain  miscroscopic  foraminifera,  and  they  sel- 
dom fail  to  contain  minute  black  shreds  of  carbona- 
ceous material.  They  also  frequently  contain  con- 
cretions ("boulders")  of  clay-iron-stone  or  even  con- 
tinuous strata  of  this  nature.  The  sandstones  range 
into  occasional  fine  conglomerates.  They  may  be 
open-textured  or  compactly  cemented  with  lime. 
Some  are  petroliferous,  as  at  Electra,  Moran,  and 
Strawn.  As  a  rule  the  sands  are  moderately  fine 
and  the  grains  angular.  Most  water  found  in  Penn- 
sylvanian  sands  is  salty.  The  limestones  present 
great  variations.  In  thickness  they  range  from  a 
few  inches  to  hundreds  of  feet.  They  are  in  rare 
cases  dolomitic.  Generally  they  consist  to  a  con- 
siderable extent  of  organic  fragments.  In  most  of 
them  may  be  found  Fusulinas  and  especially  seg- 
ments of  crinoid  stems,  which  can  quite  readily  be 
found  and  noted.  These  are  common  also  in  many 
shales  and  clays  of  the  Pennsylvanian.  The  color  is 
varied  but  mostly  gray.  Some  are  sandy,  some 
shaly.  In  the  thickest  limestones,  chert  and  flint 
are  common  at  some  horizons. 

THE  BEND  FORMATION 

This  is  the  deep  formation  which  is  oil-bearing 
in  the  Ranger  field.  It  is  possibly  of  Pottsville  age, 
and  clearly  lies  near  the  middle  of  what  is  gener- 
ally called  the  Carboniferous  strata.  It  is  known 
to  outcrop  in  McCulloch,  San  Saba,  Mason,  Gillespie, 
Blanco,  and  Burnet  counties,  and  underlies  the  later 
Carboniferous  to  the  north  of  this  region  and  no 
doubt  in  many  places  underlies  the  Comanchean  and 
possibly  also  later  Carboniferous  strata  west  and 
south  of  the  Central  Mineral  Region.  It  is  likely 


54  Handbook  of  Aids  to 

enough  that  the  Bend  is  to  be  correlated  with  the 
Dimple  formation  in  Brewster  County,  which  is 
Pennsylvanian  in  age. 

The  Bend  formation  consists  mostly  of  limestone 
but  with  this  is  also  some  shale.  One  of  the  shaly 
members  occurs  in  the  limestone.  The  other  forms 
the  basal  member  of  the  formation.  Part  of  the 
limestone  and  nearly  all  of  the  shale  is  dark,  almost 
black.  Much  of  this  limestone  can  be  told  from  later 
limestones  in  the  Pennsylvanian  by  its  somewhat 
more  developed  crystallization.  When  observed  in 
thin  sections  this  crystallization  is  seen  to  give  the 
rock  a  blotched  or  streaked  appearance,  with  irreg- 
ularly shaped  and  distributed  areas  of  crystalline 
and  granular  tracts.  The  largest  microscopic  cal- 
cite  crystals  have  a  decided  tendency  to  radiate  from 
what  appears  to  be  microscopic  organic  centers. 
Thin  microscopic  sections  of  this  rock  are  rarely 
entirely  free  from  spicules  of  sponges  and  in  some 
layers  these  are  present  in  profusion.  The  forma- 
tion also  contains  some  black  chert.  Some  calca- 
reous layers  in  the  upper  shale  are  filled  with  sponge 
spicules  and  are  quite  siliceous.  To  some  extent 
the  shales  in  this  formation  may  be  recognized  by  not 
making  as  much  fine  mud  under  the  drill  as  is  the 
case  with  most  of  the  shales  of  later  Pennsylvanian 
deposits. 

THE  MISSISSIPPIAN,  THE  DEVONIAN,  AND  THE  SILU- 
RIAN FORMATIONS 

Rocks  of  these  ages  are  of  very  little  importance 
to  drillers  in  Texas,  unless  it  should  be  found  that 
the  Bend  is  of  late  Mississippian  age,  which  can  not 
yet  be  regarded  as  demonstrated.  The  Fussulman 
limestone  in  the  Franklin  Mountains,  and  the  upper 
part  of  the  Maravillas  chert  in  the  Marathon  uplift 
probably  are  of  Silurian  age,  and  the  Caballos  no- 
vaculite  in  Brewster  County,  a  conspicuous  white 
flint,  is  probably  Devonian. 


Identification  of  Geological  Formations         55 
THE  ORDOVICIAN  FORMATIONS 

In  the  central  part  of  the  state  the  Ordovician  is 
represented  by  the  upper  part  of  the  Ellenburger 
limestone  which  is  a  hard,  fine-grained,  and  com- 
pact white  rock  that  changes  downward  into  a  some- 
what more  coarse-grained  but  still  hard  and  yellow- 
ish dolomite,  containing  some  oolitic  layers  and  some 
white  chert.  The  lower  part  of  the  Ellenburger  be- 
longs to  the  Cambrian.  The  two  make  one  continu- 
ous formation,  a  thousand  feet  thick,  without  any 
interbedded  shales  or  clays.  It  has  been  entered  by 
a  deep  boring  at  Myra,  in  Cooke  County,  where  it 
comes  up  to  within  some  2200  feet  of  the  surface, 
and  has  also  been  encountered  in  some  borings  in  a 
triangular  area  roughly  outlined  by  a  line  connect- 
ing Parker,  Lampasas,  and  McCulloch  counties, 
where  this  formation  underlies  the  Bend,  which  is 
oil-bearing.  It  outcrops  in  McCulloch,  San  Saba, 
Mason,  Llano,  Burnet  and  Gillespie  counties,  and 
no  doubt  will  be  found  underlying  later  rocks  in  the 
region  adjacent  to  these  counties.  In  Brewster 
County  rocks  of  this  age  have  been  found  in  the 
Marathon  uplift  and  contain  dark  flint  and  chert 
and  consist  of  thin-bedded,  dark,  bituminous  lime- 
stones and  shale.  The  El  Paso  limestone,  also  of 
this  age,  is  a  dolomite  1000  feet  thick  on  the  south- 
west of  the  Hueco  Mountains,  underlying  the  Huecp 
limestone  in  that  region.  The  Montoya  limestone 
near  El  Paso  is  also  Ordovician. 

THE  CAMBRIAN  FORMATIONS 

Formations  of  Cambrian  age  occur  in  San  Saba, 
Mason,  Llano,  Burnet,  Gillespie  and  adjacent  coun- 
ties, and  have  been  drilled  into  for  water  or  for 
oil  in  these  and  in  some  adjacent  counties.  Much 
of  the  Cambrian  consists  of  gray  or  red  sandstone. 
Some  of  this  is  mixed  with  lime  and.glauconite.  The 
upper  part  of  the  Cambrian  is  a  hard,  dolomitic  lime- 


56  Handbook  of  Aids  to 

stone,  the  Ellenburger  limestone  described  above. 
This  measures  several  hundred  feet  in  thickness. 
West  of  the  Pecos  the  Cambrian  is  all  sandstone. 
It  is  known  as  the  Van  Horn,  the  Bliss,  and  the 
Brewster  sandstones. 

ARCHEAN  AND  IGNEOUS  ROCKS 

Rocks  of  Archean  ages  are  mostly  granite,  gneiss 
and  schist,  and  they  occur  in  Llano,  Burnet,  Gilles- 
pie,  and  Blanco  counties.  They  have  been  entered 
under  overlying  later  rock  in  some  borings  in  these 
and  adjacent  counties.  The  schists  can  usually  be 
known  by  their  abundant  mica  and  the  granite  by 
its  pink  feldspar.  Schistose  rocks  have  been  reached 
in  some  deep  borings  at  Georgetown  and  near  San 
Antonio.  Chloritic  rocks,  and  dark  basaltic  rocks 
occur  in  Kinney,  Uvalde,  Medina,  Travis  and  Wil- 
liamson counties,  and  may  exist  also  in  Hays,  Bexar, 
and  Guadalupe  counties.  These  are  of  igneous  or- 
igin. The  former  may  be  known  by  its  dark  green 
color  and  by  its  softness,  the  latter  by  its  dark  color, 
crystalline  texture,  and  extreme  toughness  and  hard- 
ness. West  of  the  Pecos,  lavas  and  basalts  cover 
much  of  the  land.  They  are  underlain  in  many 
places  by  volcanic  tuffs,  and  sands  in  which  drilling 
operations  are  often  successfully  made  for  water,  as 
in  Jeff  Davis,  Presidio  and  Brewster  counties. 


Identification  of  Geological  Formations        57 


TABLES  FOR  IDENTIFICATION  OF 
SOME  MINERALS  AND  ROCKS 

Owing  to  the  different  chemical  and  physical  prop- 
erties of  minerals  and  of  rocks  they  may  usually  be 
recognized  by  characteristics  that  become  evident  on 
close  inspection,  or  which  appear  from  their  reac- 
tions to  some  simple  tests  to  which  they  may  be 
submitted.  Some  of  these  tests  were  known  to  the 
ancients  and  are  practiced  in  various  trades  today. 
Some  of  the  processes  involved  are  fundamental  to 
great  industries.  Non-magnetic  iron  ores  change 
to  iron  when  smelted,  and  then  become  magnetic. 
Carbon  dioxide  gas  is  evolved  from  carbonates,  such 
as  calcite  or  carbonate  of  soda,  when  acted  on  by 
hydrochloric  acid.  This  is  the  way  "soda  water" 
is  made.  It  is  the  same  gas  we  see  forming  when 
a  fragment  of  limestone  effervesces  in  the  acid.  Lime 
is  burned  from  limestone  and  it  slacks  in  water.  So 
does  a  small  fragment  of  the  same  rock,  when  heated 
to  redness  and  then  placed  in  a  drop  of  water. 

By  taking  advantage  of  these  and  other  well 
known  processes,  trying  first  one  and  then  another, 
it  is  possible  to  correctly  identify  all  of  the  more 
common  sedimentary  rocks.  They  involve  the  fund- 
amental rudiments  of  determinative  mineralogy.  To 
aid  the  beginner  some  tables  have  here  been  pre- 
pared, showing  characteristics  of  different  materials 
as  they  appear  to  the  unaided  eye  or  under  a  common 
magnifying  glass,  and  also  some  characteristic  reac- 
tions which  minerals  and  rocks  show  when  subjected 
to  a  few  tests.  These  examinations  and  tests  are 
as  follows: 


58  Handbook  of  Aids  to 

1 . .  Observation  of  general  appearance. 

2.  .Examination  of  texture. 

3 . .  Observation  of  color. 

4 . .  Observation  of  streak. 

5 . .  Test  for  hardness. 

6.. Acid  test. 

7.  .Test  for  fumes. 

8.. Fire  test. 

To  make  the  required  observations  it  is  not  nec- 
essary to  have  much  apparatus.  What  little  is  need- 
ed can  be  obtained  at  very  small  expense  and  effort. 
The  following  will  suffice: 

1 . .  A  small  magnifying  glass,  such  as  a  linen  tester, 
which  can  be  bought  in  any  jewelry  store,  or 
from  any  dealer  in  optical  goods. 

2 . .  An  acid  bottle  filled  with  a  twenty  per  cent  solu- 
tion of  hydrochloric  acid.  The  bottle  should 
have  a  glass  stopper. 

3 . .  A  pocket  knife. 

4 . .  A  "tin"  spoon,  or  a  thin  sheet  of  iron,  as  from 
a  "tin"  can. 

5 . .  A  piece  of  broken  window  glass. 

6 . .  A  small  horseshoe  magnet. 

7 . .  Pieces  of  broken  porcelain  or  chinaware. 

In  case  of  most  materials  it  will  not  be  necessary 
to  make  all  of  the  observations  and  tests  described. 
One  or  another  may  be  made  first,  according  to  the 
judgment  of  the  observer.  In  every  case,  when  a 
conclusion  has  been  reached,  the  brief  description 
of  the  identified  rock  or  mineral  found  in  this  text 
should  be  consulted  before  the  identification  is 
recorded. 

A  practice  has  recently  been  resorted  to  by  in- 
experienced drillers  who  record  some  materials  in 
the  log  merely  as  "rock.*  This  gives  no  ^clue  at  all 
to  the  nature  of  the  material  so  described,  except 
perhaps  that  it  is  harder  than  the  materials  above 
and  below  it.  The  fact  is  that  all  formations  con-^ 
sist  of  rock:  sand  and  clay  are  rock,  as  well  as 
sandstone,  limestone  or  granite.  Evidently  the  term 
is  used  when  the  log-maker  is  unable  to  tell,  or  too 


Identification  of  Geological  Formations        59 

hurried  to  note  whether  the  "rock"  is  limestone  or 
sandstone.  Sometimes  inability  to  make  the  neces- 
sary distinction  is  quite  excusable  as  in  case  of  a 
cemented  sandstone  of  fine  texture.  "Hard  rock, 
undetermined/'  would  be  a  more  proper  record  in 
such  a  case.  A  sandstone  cemented  with  calcareous 
material  will  effervesce  in  acid,  but  if  a  few  frag- 
ments be  crushed  and  treated  with  a  sufficient  quan- 
tity of  acid  until  effervescence  ceases,  the  quartz 
grains  of  the  sand  will  remain  and  can  be  seen 
through  a  magnifying  glass. 


60 


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Identification  of  Geological  Formations         63 


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Serpentine 


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Glauconjte 
Serpentine 
Hornblende. 


Identification  of  Geological  Formations         65 


us.!!!! 


66 


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Identification  of  Geological  Formations        67 


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Gypsum 
Barite 
Pyrite 
Marcasite. 


Identification  of  Geological  Formations         69 


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Clay-iron-stoi 
Hematite 
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Limestone 
Dolomite. 


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NOTICE 

Headers  of  this  text,  interested  in  the 
geology  of  Texas,  will  find  it  advantageous 
to  have  at  hand  for  reference  Bulletin  44 
of  the  University  of  Texas*  This  has 
been  prepared  by  the  Bureau  of  Economic 
Geology,  and  Is  a  review  of  the  geology  of 
the  state  as  far  as  it  has  been  made  out  to 
the  present  time.  It  contains-  maps  and 
sections  for  ready  reference  and  clear  pre- 
sentation of  the  known  surflcial  distribution 
and  the  thickness  and  nature  of  the  forma- 
tions described  on];  briefly  in  this  .text. 
Bulletin  44  will  be  mailed  promptly  on  the 
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Address  Je  A.  Udden,  Bureau  of  Eco- 
nomic Geology,  Austin,  Texas* 


